dt-bindings: pinctrl: add binding for Ralink MT7621 pinctrl

Add binding for the Ralink MT7621 pin controller for MT7621 SoC.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20220414173916.5552-13-arinc.unal@arinc9.com


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