arm64: dts: renesas: r8a779a0: Add INTC-EX device node

Populate the device node for the Interrupt Controller for External
Devices (INTC-EX) on R-Car V3U, which serves external IRQ pins IRQ[0-5].

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/4e2297e1066df483c0434df487df5b79e76b75b8.1624460378.git.geert+renesas@glider.be