drm: exynos: Add driver for HDMI audio interface

The hdmi-codec interface added in this patch is required to properly
support HDMI audio. Currently the audio part of the SoC internal
HDMI transmitter is configured with fixed values, which makes HDMI
audio working by chance, only on boards having an external audio
codec connected in parallel with the HDMI audio transmitter's input
I2S interface.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>