pinctrl: mediatek: add EINT support to MT7622 SoC

Add EINT support to MT7622 SoC and the support is made as just an option
to MT7622 pinctrl.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>