ARM: mediatek: Add MT2701 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. Add MT2701.

Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>