pinctrl: uniphier: support per-pin input enable for new SoCs

Upcoming new pinctrl drivers for PH1-LD11 and PH-LD20 support input
signal gating for each pin.  (While, existing ones only support it
per pin-group.)  This commit updates the core part for that.

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