ASoC: dt-bindings: Convert SPDIF Transmitter binding to a schema

ASoC: dt-bindings: Convert SPDIF Transmitter binding to a schema

The SPDIF Transmitter binding is used by Linux with a matching Device
Tree binding.

Now that we have the DT validation in place, let's convert the device
tree bindings for that driver over to a YAML schema.

Cc: alsa-devel@alsa-project.org
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210901091852.479202-4-maxime@cerno.tech
Signed-off-by: Mark Brown <broonie@kernel.org>