arm64: dts: meson: libretech-ac: add initial audio playback support

Add initial audio support limited to HDMI i2s.

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/20200421163935.775935-6-jbrunet@baylibre.com