dt-bindings: irqchip: renesas-irqc: Document r8a774b1 bindings

Document SoC specific bindings for RZ/G2N (r8a774b1) SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
