ASoC: Add sun8i digital audio codec

Add the sun8i audio codec which handles the digital register of
A33 codec.
The driver handles only the basic playback from the DAC to headphones.
All other features (microphone, capture, etc) will be added later.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>