arm64: mediatek: Select PINCTRL for Mediatek platform

MediaTek SoC expect to work with a pinctrl driver.
Select PINCTRL if ARCH_MEDIATEK is selected.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>