LoongArch: Update dts for Loongson-2K2000 to support PCI-MSI

Current dts file for Loongson-2K2000 misses the interrupt-controller &
interrupt-cells descriptions in the msi-controller node, and misses the
msi-parent link in the pci root node. Add them to support PCI-MSI.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>