ARM: mediatek: Add config options for mediatek SoCs.

The upcoming MTK pinctrl driver have a big pin table for each SoC
and we don't want to bloat the kernel binary if we don't need it.
Add config options so we can build for one SoC only.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>