Soft S/PDIF decoder

This is a software S/PDIF decoder for the STM32L476G-Discovery board.

Wiring:

 - Connect the input signal to PA0 and PE13
 - Connect PA1 to PA5
 - Connect PA3 to PE15

This was originally going to output the audio over the DAC but due to the
available GPIO pins I abandoned that plan. The S/PDIF decoding works though.