ASoC: sunxi: Add support for the SPDIF block

The sun4i, sun5i and sun7i SoC families have an SPDIF
block which is capable of playback and capture.

This patch enables the playback of this block for
the sun4i families.

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>