pinctrl: mediatek: Add EINT support to MTK pinctrl driver.

MTK SoC support external interrupt(EINT) from most SoC pins.
Add EINT support to pinctrl driver.

Signed-off-by: Maoguang Meng <maoguang.meng@mediatek.com>
Signed-off-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>