dt-bindings: irqchip: renesas-irqc: Add R-Car M3-W+ support

Document support for the Interrupt Controller for External Devices
(INT-EC) in the Renesas R-Car M3-W+ (r8a77961) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/55d2c30cb14b2e10193a7fd4aa7670c70f360037.1622546880.git.geert+renesas@glider.be