ARM: dts: tango4: remove bogus interrupt-controller property

dtc points out that the parent node of the interrupt controllers is not
actually an interrupt controller itself, and lacks an #interrupt-cells
property:

arch/arm/boot/dts/tango4-vantage-1172.dtb: Warning (interrupts_property): Missing #interrupt-cells in interrupt-parent /soc/interrupt-controller@6e000

This removes the annotation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>