ASoC: pcm186x: Add PCM186x binding documentation

Add the dt-binding documentation for the TI PCM186x 2ch and 4ch Audio
ADCs With Universal Front End.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>