ASoC: meson: g12a: add toacodec dt-binding documentation

Add the DT bindings and documentation of the internal audio DAC glue found
on Amlogic g12a and sm1 SoC families

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200221153607.1585499-2-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>