ARM: dts: uniphier: add syscon property for UniPhier sound system

This patch adds syscon property for specifying soc-glue core into
device-tree of PXs2 SoC.

Currently, soc-glue core is used for changing the state of S/PDIF
signal output pin to signal output state or Hi-Z state.

Signed-off-by: Katsuhiro Suzuki <suzuki.katsuhiro@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>