Access to nRF528x hardware decoder

### Subject of the issue
I'd like to gain access to the hardware decoder inside the nrF528x, right now I'm reading a quadrature encoder using interrupts on pins 4 (for channel A) and 6 (for channel B), using the InterruptIn mbed instructions, starting from a mbed library found on mbed site, and adapting it to work under arduino core, basically by adding mbed:: before mbed instructions and some callbacks.

```
mbed::InterruptIn sqwA;
mbed::InterruptIn sqwB;
mbed::InterruptIn pulZero;
```

```
sqwA.rise(mbed::callback(this, &QEI::encode));
sqwA.fall(mbed::callback(this, &QEI::encode));
sqwB.rise(mbed::callback(this, &QEI::encode));
sqwB.fall(mbed::callback(this, &QEI::encode));
pulZero.rise(mbed::callback(this, &QEI::index));
```
The code works great for frequencies under 7 kHz, pulse count is correct and also frequency and speed calculations, but when i increase the motor speed i start to lose pulses and the count (and also frequency and speed calculation) goes wrong. So I wanted to see if the problem was the interrupt frequency, maybe is too slow for that motor speed, and I thought that using the hardware decoder was a good option because in theory it gives me the pulse count without any software calculation on the main loop so it should be the fastest option. I started from the example on the nordic SDK 15.0 for the QDEC and a wrote a little sketch to see if I was able to compile it:

```
#include
#include
#include "nrfx_gpiote.h"
#include
#include
#include

#define NRFX_QDEC_ENABLED 1
#define NRFX_QDEC_CONFIG_REPORTPER NRF_QDEC_REPORTPER_120
#define NRFX_QDEC_CONFIG_SAMPLEPER NRF_QDEC_SAMPLEPER_256us
#define NRFX_QDEC_CONFIG_PIO_A (4u)
#define NRFX_QDEC_CONFIG_PIO_B (6u)
#define NRFX_QDEC_CONFIG_PIO_LED 0xFFFFFFFF
#define NRFX_QDEC_CONFIG_LEDPRE 0
#define NRFX_QDEC_CONFIG_LEDPOL NRF_QDEC_LEPOL_ACTIVE_LOW
#define NRFX_QDEC_CONFIG_IRQ_PRIORITY 2
#define NRFX_QDEC_CONFIG_DBFEN NRF_QDEC_DBFEN_DISABLE
#define NRFX_QDEC_CONFIG_SAMPLE_INTEN NRF_QDEC_INT_SAMPLERDY_MASK

static volatile uint32_t m_accdblread;
static volatile int32_t m_accread;

nrfx_qdec_config_t cfg =
{
cfg.reportper = NRFX_QDEC_CONFIG_REPORTPER,
cfg.sampleper = NRFX_QDEC_CONFIG_SAMPLEPER,
cfg.psela = NRFX_QDEC_CONFIG_PIO_A,
cfg.pselb = NRFX_QDEC_CONFIG_PIO_B,
cfg.pselled = NRFX_QDEC_CONFIG_PIO_LED,
cfg.ledpre = NRFX_QDEC_CONFIG_LEDPRE,
cfg.ledpol = NRFX_QDEC_CONFIG_LEDPOL,
cfg.dbfen = NRFX_QDEC_CONFIG_DBFEN,
cfg.sample_inten = NRF_QDEC_INT_SAMPLERDY_MASK,
cfg.interrupt_priority = NRFX_QDEC_CONFIG_IRQ_PRIORITY};

nrfx_qdec_config_t *cfgPoint = &cfg;

static void gpio_config(void)
{
nrf_gpio_cfg_input(NRFX_QDEC_CONFIG_PIO_A, NRF_GPIO_PIN_PULLUP);
nrf_gpio_cfg_input(NRFX_QDEC_CONFIG_PIO_B, NRF_GPIO_PIN_PULLUP);
}

static void qdec_event_handler(nrfx_qdec_event_t event)
{
if (event.type == NRF_QDEC_EVENT_REPORTRDY)
{
m_accdblread = event.data.report.accdbl;
m_accread = event.data.report.acc;
nrfx_qdec_disable();
}
}

static void qdec_config(void)
{
uint32_t err_code;

// Initialize hardware
err_code = nrfx_qdec_init(cfgPoint, qdec_event_handler);
APP_ERROR_CHECK(err_code);
Serial.println("QDEC testing started");

nrfx_qdec_enable(); // Event and corresponding interrupt are enabled.
Serial.println("nrf_drv_qdec_enable");
}

void setup()
{
Serial.begin(9600);
gpio_config();
qdec_config();
}

void loop()
{
Serial.println(m_accdblread);
Serial.println(m_accread);
delay(100);
}
```

But the problem as commented on arduino/ArduinoCore-nRF528x-mbedos#58, I get the compiler error:

```
c:\Users\a.rondon\Documents\Arduino\Arduino_VSC\nano33BLE_hardwareDecoder/nano33BLE_hardwareDecoder.ino:65: undefined reference to `nrfx_qdec_init'

c:\Users\a.rondon\Documents\Arduino\Arduino_VSC\nano33BLE_hardwareDecoder/nano33BLE_hardwareDecoder.ino:69: undefined reference to `nrfx_qdec_enable'
```

and as stated on arduino/ArduinoCore-nRF528x-mbedos#58, the src files for `nrfx_qdec.h` are missing, I tried copying the files from nordic SDK 15 directly on the sketch folder as suggested by @facchinm but that didn't work either, I took a look at arduino/ArduinoCore-nRF528x-mbedos#72 and what @Polldo said here https://github.com/arduino/ArduinoCore-nRF528x-mbedos/issues/58#issuecomment-625179806, but I think I'm using the right file `#include `

So, if there is something I could try to solve this problem? There are some mbed API's to read the decoder directly??

Thanks for all the help you could provide guys!!

### My workbench
* Arduino nano 33 BLE sense
* Board version V2.0, with MCU nRF52840 (32-bit ARM® Cortex™-M4 CPU)
* Encoder connected to pins P1_14 and P1_15 (D4 and D6)
* Using VsCode with arduino extension

### Expected behavior
Well I'd like to use the hardware decoder inside the arduino nano 33 BLE

### Actual behavior
The code doesn't compile because it can find the source files for nrfx_qdec.h

Start with the nrfx_qdec.h usage in the provided sketch and the missing source-file discussion in ArduinoCore-nRF528x-mbedos#58, then compare it with the Nordic SDK 15.0 QDEC example and issue #72. The work is done when the sketch links successfully and can access the nRF52840 hardware decoder on the stated encoder pins.

Written by the indexing model from the issue text.