dt-bindings: irqchip: renesas-irqc: Document r8a77470 support

Renesas RZ/G SoC have the R-Car gen2 compatible IRQC interrupt
controllers. Document RZ/G1C (also known as R8A77470) SoC bindings.

Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>