OMAPDSS: hdmi4: Register ASoC platform device for omap hdmi audio

Implements callbacks for OMAP HDMI audio platform driver and registers
it in probe function. Unregistering is done in remove function.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>