ASoC: dt-bindings: add dt bindings for adau1977 audio codec

Add device-tree bindings documentation file for ADAU1977 audio codec.
This describes device-tree fields that are already supported by the driver.

The driver supports both I2C AND and SPI, and this doc covers both aspects
of the DT configuration.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Mark Brown <broonie@kernel.org>