pinctrl: mediatek: Add initial pinctrl driver for MT6797 SoC

Add initial pinctrl driver for Mediatek MT6797 SoC supporting only
GPIO and pinmux configurations.

Tested-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>