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

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

Similar to commit 98a9e2ac3755 ("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: 24af105962c8 ("MIPS: Loongson64: DeviceTree for LS7A PCH")
Tested-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>