arm64: dts: renesas: r8a77990: Add INTC-EX device node

This patch adds a device node for the Interrupt Controller for External
Devices (INTC-EX) on R-Car E3, which serves external IRQ pins IRQ[0-5].

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>