ASoC: add S/PDIF support to Armada 370 DB ASoC driver

The Armada 370 DB board not only has analog audio input/output, but
also S/PDIF input/output. This commit adds support for S/PDIF in the
ASoC machine driver of the Armada 370 DB platform, and adjusts the
Device Tree bindings documentation accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@linaro.org>