pinctrl: uniphier: rework UniPhier pinctrl entries in Kconfig

There is a plan to support more pinctrl drivers for this SoC family.
Move the driver entries into a sub menu by using "menuconfig".
Also, add the missing dependency "depends on OF && MFD_SYSCON".

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