ASoC: adau1372: Add bindings documentation

Add device tree binding documentation for Analog Devices ADAU1372.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201127123030.1610574-1-alexandre.belloni@bootlin.com


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