ASoC: meson: add g12a tohdmitx control

Add support for the hdmitx control glue of the Amlogic g12a SoC family.
This glue links the 3 TDM and 2 SPDIF output interfaces of the SoC to
the related inputs of the Synopsys HDMI controller found in these SoCs.

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>