dt-bindings: pinctrl: add external interrupt support to MT7622 pinctrl

Extend the capability of MT7622 pinctrl with adding EINT so that each
GPIO can be used to notify CPU when a signal state is changing on the
line as an external interrupt.

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