Support Raspberry Pi 4

[mill1000]

The Raspberry Pi 4B has enough changes that the project is currently incompatible. Some discussion has occurred in #8.

At least the following issues exists

• Selected DMA channel is in use on the Pi 4B.
• PLLD frequency was increased from 500 MHz to 750 MHz.

Currently we assumed a fixed PLLD frequency when calculating the divisor:

raspdif/source/main.c

Line 199 in facd46e

double divisor = (500e6 / spdif_clock);