dt-bindings: irqchip: renesas-irqc: Document R-Car M3-W, V3M, D3 support

Document support for the Interrupt Controller for Externel Devices
(INTC-EX) in the Renesas M3-W (r8a7796), V3M (r8a77970), and D3
(r8a77995) SoCs.

No driver update is needed.

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