pinctrl: add mt7981 pinctrl driver

Add pinctrl driver for the MediaTek MT7981 SoC, based on the driver
which can also be found the SDK.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/ef5112946d16cacc67e65e439ba7b52a9950c1bb.1674693008.git.daniel@makrotopia.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>