dt-bindings/irqchip/renesas-irqc: Document R-Car M3-N support

Document support for the Interrupt Controller for Externel Devices
(INTC-EX) in the Renesas M3-N (r8a77965) SoC.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-renesas-soc@vger.kernel.org
Cc: Rob Herring <robh+dt@kernel.org>
Link: https://lkml.kernel.org/r/1519658712-22910-1-git-send-email-geert%2Brenesas@glider.be