Update with some fixes for 3B mclk phase issue

[xhuw]

Add to readme to explain the issue and the manual steps required to have a well behaving pi

Update setup.sh so it should work in the 48kHz by default (before the phase issue was present at boot)

C file to check the clocks on an rpi 3b: ed806993b9d932fcf66cb0a9fdd78a57-491848da9419e3130bb3a12f1a69950df3faf8d4.zip

cc -g minimal_clk.c
sudo a.out -c3 -r

minimal_clk.c does not work for pi 4B

https://github.com/xmos/sw_xvf3800/issues/134

[xhuw]

@lucianomartin updated setup.sh, I don't mind removing those changes if you would rather do it (I haven't tested the avs stuff)