ASoC: uniphier: add syscon property for UniPhier sound system

This patch adds syscon property for specifying soc-glue core.

Currently, soc-glue core is used for changing the state of S/PDIF
signal output pin to signal output state or Hi-Z state. After
resetting of SoC Hi-Z state is selected. This driver set to signal
output state when syscon property is available.

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