drm/bridge: sii902x: Implement HDMI audio support

Implement HDMI audio support by using ASoC HDMI codec. The commit
implements the necessary callbacks and configuration for the HDMI
codec and registers a virtual platform device for the codec to attach.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ef6895d016554b1e0c8b388d4993bcd6a3699341.1558964241.git.jsarha@ti.com