dt-bindings: pinctrl: add bindings for MT7981 SoC

Add bindings for the MT7981 pinctrl driver. As MT7981 has most features
in common with MT7986 (but has a different layout in terms on pinctrl
and clocks), the existing mediatek,mt7986-pinctrl.yaml was used as an
example to create a similar document covering MT7981.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/3f0fd0becc338eef66caeb7244c3c432b8d1ef7a.1674693008.git.daniel@makrotopia.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>