ASoC: uniphier: add syscon property to binding document

This patch adds syscon property for specifying soc-glue core into
DT binding documentation.

Currently, soc-glue core is used for changing the state of S/PDIF
signal output pin to signal output state or Hi-Z state. If you don't
need to use features on soc-glue described above you can ommit this
property.

Signed-off-by: Katsuhiro Suzuki <suzuki.katsuhiro@socionext.com>
Signed-off-by: Mark Brown <broonie@kernel.org>