arm64: dts: renesas: r8a77965: Add #interrupt-cells property

Add "#interrupt-cells" property and "interrupt-controller" label to
"interrupt-controller@e61c0000" device node.

This silences the following DTC compiler warnings:
Warning (interrupts_property): Missing interrupt-controller or
interrupt-map property in /soc/interrupt-controller@e61c0000
Warning (interrupts_property): Missing #interrupt-cells in
interrupt-parent /soc/interrupt-controller@e61c000

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>