dt-bindings: gpio: rcar: Convert to json-schema

Convert the Renesas General-Purpose Input/Output Ports (GPIO) Device
Tree binding documentation to json-schema.

Drop the deprecated "renesas,gpio-rcar" compatible value.
Document missing properties.
Drop the first example, as it doesn't add much value.

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