dt-bindings: irqchip: renesas-intc-irqpin: Convert to json-schema

Convert the Renesas Interrupt Controller (INTC) for external pins Device
Tree binding documentation to json-schema.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Co-developed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[robh: drop allOf]
Signed-off-by: Rob Herring <robh@kernel.org>