ASoC: sunxi: Add bindings for sun8i to SPDIF

The H3 SoC uses the same SPDIF block as found in earlier SoCs, but the
transmit fifo is at a different address.

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>