dt-bindings: irqchip: renesas-irqc: Add R-Car V3U support

Document support for the Interrupt Controller for External Devices
(INT-EC) in the Renesas R-Car V3U (r8a779a0) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/85b246cc0792663c72c1bb12a8576bd23d2299d3.1643200256.git.geert+renesas@glider.be