PwmOut on K64F does not work

mbed Classic (works):

``` cpp
#include "mbed.h"
PwmOut led(D3);

int main() {
while(1) {
for(float p = 0.0f; p < 1.0f; p += 0.1f) {
led = p;
wait(0.1);
}
}
}
```

mbed OS, doesnt work:

``` cpp
static PwmOut led(D3);
float a = 0.0f;
static void blinky(void) {
a += 0.1f;
if (a > 1.0f) a = 0.0f;

led = a;
}

void app_start(int, char**) {
get_stdio_serial().baud(115200);
minar::Scheduler::postCallback(blinky).period(minar::milliseconds(100));
}
```

This is kind of a real problem for me, as a bunch of things I need to demo on Monday require PWM...

Start by reproducing the mbed OS example using PwmOut led(D3), app_start, and the blinky callback on a K64F board, then compare its behavior with the working mbed Classic example. Trace the PWM driver and K64F pin mapping involved in D3. Done means the demonstrated PWM output changes as the callback updates led.

Written by the indexing model from the issue text.