ASoC: sunxi: Add the Allwinner A10 codec bindings

The Allwinner SoCs have an in-SoC audio controller taking the role of a DAI
and a codec.

Add the binding documentation for that controller on the A10.

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