arm64: dts: r8a77970: Add INTC-EX device node

Add a device node for the Interrupt Controller for External Devices
(INTC-EX) on R-Car V3M, 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>