pinctrl: mediatek: Add MT8516 Pinctrl driver

This commit adds the pinctrl driver for the MediaTek's MT8516 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>