pinctrl: mediatek: add MT7623 pinctrl driver based on generic pinctrl binding

Adding MT7623 pinctrl driver based on generic pinctrl binding, that is
good example and demonstrates how to port any other MediaTek SoCs
pinctrl-moore core when people really would like to use the generic
pinctrl binding to support these MediaTek SoCs.

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