arm64: dts: renesas: r8a779f0: Add INTC-EX node

Add the device node for the Interrupt Controller for External Devices
(INTC-EX) on the Renesas R-Car S4-8 (R8A779F0) SoC, 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>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/8f5612c0353b8c90f98366978563340d93c7ae58.1690447013.git.geert+renesas@glider.be