MIPS: Loongson64: DTS: Fix msi node for ls7a

Add it to silent warning:
arch/mips/boot/dts/loongson/ls7a-pch.dtsi:68.16-416.5: Warning (interrupt_provider): /bus@10000000/pci@1a000000: '#interrupt-cells' found, but node is not an interrupt provider
arch/mips/boot/dts/loongson/loongson64g_4core_ls7a.dts:32.31-40.4: Warning (interrupt_provider): /bus@10000000/msi-controller@2ff00000: Missing '#interrupt-cells' in interrupt provider
arch/mips/boot/dts/loongson/loongson64g_4core_ls7a.dtb: Warning (interrupt_map): Failed prerequisite 'interrupt_provider'
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/loongson/loongson64g_4core_ls7a.dts

@@ -33,6 +33,7 @@ msi: msi-controller@2ff00000 {
 		compatible = "loongson,pch-msi-1.0";
 		reg = <0 0x2ff00000 0 0x8>;
 		interrupt-controller;
+		#interrupt-cells = <1>;
 		msi-controller;
 		loongson,msi-base-vec = <64>;
 		loongson,msi-num-vecs = <192>;