ASoC: sunxi: Add A10 I2S controller binding documentation

Introduce the device tree binding for the I2S controller found in the
Allwinner A10 and later SoCs.

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