pinctrl: mediatek: use pin descriptor all in pinctrl-mtk-common-v2.c

all use pin descriptor instead in pinctrl-mtk-common-v2.c for the
consistency and extensibility.

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