arm64: dts: r8a7795: Add INTC-EX device node

Add a single r8a7795 INTC-EX device node to support
external IRQ pins IRQ0 -> IRQ5.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>