bcm2835: implement channel map API

Report all layouts supported by the HDMI protocol to userspace.
Make the videocore set the correct layout according to the
userspace request.

Some code taken from patch_hdmi.c. In particular, the HDMI channel
layout table was copied without changes - with the idea in mind that
hopefully it can be shared one day. Or at least updating it will be
simpler.

In my tests, everything appears to work, except when outputting
FL FR RL RR. Then my receiver outputs RL on both the RL and RR
speakers, while RR is never heard.