bcm2835: add fallback channel layouts if channel map API is not used

Should be more useful than just forcing stereo.

We can't match the exact legacy ALSA channel layouts, so this is a
"best effort" hack.

I'm not sure what happens if the application requests channel counts
that are not power-of-2s. The channel map API hopefully forces
applications which use the channel map API to request the correct
count by adding padding channels, but the bare API enforces
nothing. Possibly this could be added to rate_hw_constraint_channels
at a later point.