ARM: dts: uniphier: make compatible of syscon nodes SoC-specific

These hardware blocks are SoC-specific, so their compatible strings
should be SoC-specific as well.  This change has no impact on the
actual behavior since it is controlled by the generic "simple-mfd",
"syscon" compatible strings.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>