arm64: dts: layerscape: add msi-cell = <1> for gic its

arm64: dts: layerscape: add msi-cell = <1> for gic its

Add msi-cell = <1> for GIC ITS. msi-parent have to be kept because it is
checked by U-Boot due to historical reasons to fix up msi-map.

Fix below CHECK_DTBS warning:
arch/arm64/boot/dts/freescale/fsl-ls1028a-kontron-kbox-a-230-ls.dtb: interrupt-controller@6000000: msi-controller@6020000: '#msi-cells' is a required property
        from schema $id: http://devicetree.org/schemas/interrupt-controller/arm,gic-v3.yaml

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>