ASoC: dt-bindings: rt9120: Add initial bindings

Add initial bindings for Richtek rt9120 audio amplifier.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1634088519-995-2-git-send-email-u0084500@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>