ASoC: meson: aiu: add audio output dt-bindings

Add the dt-bindings and documentation of the AIU audio controller.
This component provides most of the audio outputs found on the Amlogic
Gx SoC family.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200213155159.3235792-4-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>