arm64: dts: meson: g12a: enable hdmi_tx sound dai provider

At the moment the sysnopsys hdmi i2s driver provides a single playback
DAI. Add the corresponding sound-dai-cell to the hdmi device node.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>