dt-bindings: irqchip: renesas-irqc: Document R-Car E3 support

Document support for the Interrupt Controller for External Devices
(INTC-EX) in the Renesas E3 (r8a77990) SoC.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>