arm64: dts: renesas: r8a774a1: Add INTC-EX device node

Add support for the Interrupt Controller for External Devices
(INTC-EX) on RZ/G2M.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>