drm: bridge: it66121: Add audio support

Adding the audio support on the HDMI bridge for I2S only.

Signed-off-by: Nicolas Belin <nbelin@baylibre.com>
Signed-off-by: Andy.Hsieh <Andy.Hsieh@mediatek.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220316135733.173950-4-nbelin@baylibre.com