dt-bindings: irqchip: renesas-irqc: Document r8a774e1 bindings

Document SoC specific bindings for RZ/G2H (r8a774e1) SoC.

Signed-off-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/53317ce1bdd1d5e517122eb5c8ea0ccaa69eba3b.1635337428.git.geert+renesas@glider.be