ASoC: codecs: Add sun8i-a33 binding documentation

Add the documentation for dt-binding of the digital audio codec driver
and the audio card driver for Sun8i-a33 SoCs.

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>