ASoC: meson: add tohdmitx DT bindings

Add the bindings and the related documentation for the audio hdmitx
control glue of the Amlogic g12a SoC family

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>