arm64: dts: renesas: r8a77965: Add INTC-EX device node

Populate the device node for the Interrupt Controller for External
Devices (INTC-EX) on R-Car M3-N, which serves external IRQ pins
IRQ[0-5].

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>