pinctrl: mediatek: add drv register support to pinctrl-mtk-common-v2.c

Certain SoCs have to program DRV register to configure driving
strength so that we add it in the existing path as an option.

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>