Add mt6397 support using mediatek common pinctrl driver.

mt6397 is a PMIC, and pinctrl/GPIO is part of 6397 chip.
Pinctrl/GPIO driver should obtain regmap from PMIC,
so adding this support to common code.

Also, mt6397 is no need to support interrupt controller,
so changing common code to skip it.
Signed-off-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>