Low Power Examples for Pico - Reinit Serial and USB?

I've been trying to port some low power examples from the Pi Pico SDK so they will run using the Arduino Core. https://raspberrypi.github.io/pico-sdk-doxygen/group__hardware__clocks.html
So far I've got blink successfully working at a variety of clock speeds.

However, the USB and UART serial ports no longer seem to work at the changed clock speed.

Is there an API call in ArduinoCore to re-initialise these or is there some other way to tell the ports they have a new clock speed?

Example code demonstrating the issue
[Clock48Mhz.h](https://github.com/Workshopshed/PiPico/blob/main/Clock48Mhz/Clock48Mhz.h)

```
#include "Clock48Mhz.h"

void setup() {
// Configure for lower power usage
set_sys_clock_48mhz();

Serial.begin(9600); //USB Serial

while (!Serial) {
; // wait for serial port to connect. Needed for native USB port only
}

Serial1.begin(115200); //UART0 on pins 0 and 1
Serial.println("Serial Tester");
}

void loop() {
if(Serial1.available()){
Serial.print((char) Serial1.read());
}
if(Serial.available()){
Serial1.print((char) Serial.read());
}
delay(1);
}
```

Start with the linked Clock48Mhz.h example and reproduce the USB Serial and Serial1 failures after set_sys_clock_48mhz(). Read the ArduinoCore-mbed clock, USB, and UART initialization entry points to determine whether an existing reinitialization API is expected. Done means documenting or implementing a supported way for both ports to work after the clock change.

Written by the indexing model from the issue text.