ARM: dts: rockchip: fix rk3288 iep-IOMMU interrupts property cells

The interrupts property in the iep-IOMMU node for the rk3288 dts file has a
spurious extra cell causing a dtc warning:

Warning (interrupts_property): interrupts size is (16), expected multiple of 12 in /iommu@ff900800

Remove the extra cell.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>