arm64: dts: meson-gx: add aiu support

Add the AIU audio device to the Amlogic GX SoC family DT.
ATM, this device provides the i2s and spdif output stages and also
the hdmi and internal codec glues.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Christian Hewitt <christianshewitt@gmail.com>
Link: https://lore.kernel.org/r/20200421163935.775935-3-jbrunet@baylibre.com