dt-bindings: irqchip: renesas-irqc: Document r8a7744 support

Document RZ/G1N (R8A7744) SoC bindings.

Reviewed-by: Chris Paterson <Chris.Paterson2@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>