pinctrl: mediatek: guard sub-directory with CONFIG_PINCTRL_MTK

CONFIG_PINCTRL_MTK is more suitable than CONFIG_ARCH_MEDIATEK
to guard the drivers/pinctrl/mediatek/ directory.
(I renamed CONFIG_PINCTRL_MTK_COMMON to CONFIG_PINCTRL_MTK.)

This allows COMPILE_TEST to descend into drivers/pinctrl/mediatek
without CONFIG_ARCH_MEDIATEK define.

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