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: NSimon Horman <horms+renesas@verge.net.au>
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>