dt-bindings: irqchip: renesas-irqc: Document r8a77980 support

Document R-Car V3H (AKA R8A77980) SoC bindings.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>