ASoC: Add bindings for PCM1789

Add a device-tree binding for Texas Instrument's PCM1789 codec.
For the moment, only I2C bus is supported but SPI could be added
in future.

Signed-off-by: Mylène Josserand <mylene.josserand@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>