Allo Piano DAC boards: Initial 2 channel (stereo) support (#1645)

Add initial 2 channel (stereo) support for Allo Piano DAC (2.0/2.1) boards,
using allo-piano-dac-pcm512x-audio overlay and allo-piano-dac ALSA ASoC
machine driver.

NB. The initial support is 2 channel (stereo) ONLY!
(The Piano DAC 2.1 will only support 2 channel (stereo) left/right output,
 pending an update to the upstream pcm512x codec driver, which will have
 to be submitted via upstream. With the initial downstream support,
 provided by this patch, the Piano DAC 2.1 subwoofer outputs will
 not function.)

Signed-off-by: Baswaraj K <jaikumar@cem-solutions.net>
Signed-off-by: Clive Messer <clive.messer@digitaldreamtime.co.uk>
Tested-by: Clive Messer <clive.messer@digitaldreamtime.co.uk>