arm64: dts: renesas: r8a779g0: Add INTC-EX node

Add the device node for the Interrupt Controller for External Devices
(INTC-EX) on the Renesas R-Car V4H (R8A779G0) SoC, which serves external
IRQ pins IRQ[0-5].

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f2e5adf62a7666db7350d9596a907bc7f9e81d43.1664369015.git.geert+renesas@glider.be