pinctrl: uniphier: support 3-bit drive strength control

The new ARMv8 SoC, PH1-LD20, supports more fine-grained drive
strength control.  Drive strength of some pins are controlled by
3-bit width registers (8-level granularity).

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