pinctrl: uniphier: guard uniphier directory with CONFIG_PINCTRL_UNIPHIER
CONFIG_PINCTRL_UNIPHIER is more suitable than CONFIG_ARCH_UNIPHIER
to guard the drivers/pinctrl/uniphier directory.
The current CONFIG_PINCTRL_UNIPHIER_CORE is a bit long
(it would break the indentation in drivers/pinctrl/Makefile),
so rename it into CONFIG_PINCTRL_UNIPHIER.