ASoC: sunxi: add sun4i SPDIF transceiver

Add devicetree bindings for the SPDIF transceiver found on
found on Allwinners A10 and A20 SoCs.

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