dt-bindings: pinctrl: renesas,rza2-pinctrl: Convert to json-schema

Convert the Renesas RZ/A2 combined Pin and GPIO controller Device Tree
binding documentation to json-schema.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200626143638.16512-1-geert+renesas@glider.be