MIPS: Loongson64: Add missing '#interrupt-cells' for loongson64c_ls7a

Similar to commit 98a9e2a ("MIPS: Loongson64: DTS: Fix msi node for ls7a").

Fix follow warnings:
  arch/mips/boot/dts/loongson/loongson64c_4core_ls7a.dts:28.31-36.4: Warning (interrupt_provider): /bus@10000000/msi-controller@2ff00000: Missing '#interrupt-cells' in interrupt provider
  arch/mips/boot/dts/loongson/loongson64c_4core_ls7a.dtb: Warning (interrupt_map): Failed prerequisite 'interrupt_provider'

Fixes: 24af105 ("MIPS: Loongson64: DeviceTree for LS7A PCH")
Tested-by: WangYuli <[email protected]>
Signed-off-by: WangYuli <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Bogendoerfer <[email protected]>

Author: WangYuli

arch/mips/boot/dts/loongson/loongson64c_4core_ls7a.dts

@@ -29,6 +29,7 @@
 		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 = <64>;