pinctrl: mediatek: add EINT support to virtual GPIOs

Virtual gpio only used inside SOC and not being exported to outside SOC.
Some modules use virtual gpio as eint and doesn't need SMT.
So this patch add EINT support to virtual GPIOs.

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