pinctrl: mediatek: cleanup for placing all drivers under the menu

Since lots of MediaTek drivers had been added, it seems slightly better
for that adding cleanup for placing MediaTek pinctrl drivers under the
independent menu as other kinds of drivers usually was done.

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