dt-bindings: pinctrl: uniphier: add UniPhier pinctrl binding

The driver has been in the tree for a while, but its binding document
is missing.  Hence, here it is.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>