dt-bindings: mt7621-gpio: convert bindings to YAML format

Convert the mt7621-gpio device tree bindings to the new YAML format.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201207081151.7489-1-sergio.paracuellos@gmail.com


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