pinctrl: uniphier: allow to have pinctrl node under syscon node

Currently, the UniPhier pinctrl driver itself is a syscon, but it
turned out much more reasonable to make it a child node of a syscon
because our syscon node consists of a bunch of system configuration
registers, not only pinctrl, but also phy, and misc registers.
It is difficult to split the node.  This commit allows to migrate to
the new DT structure.

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