dt-bindings: PCI: loongson: Document msi-parent property

dt-bindings: PCI: loongson: Document msi-parent property

Loongson PCI controllers found in LS2K1000/2000 SoCs
(loongson,ls2k-pci), 7A1000/2000 bridge chips (loongson,ls7a-pci), and
RS780E bridge chips (loongson,rs780e-pci) all have their paired MSI
controllers.

Though only the one in LS2K2000 SoC is described in devicetree, we
should document the property for all variants. For the same reason, it
isn't marked as required for now.

Fixes: 83e757ecfd5d ("dt-bindings: Document Loongson PCI Host Controller")
Signed-off-by: Yao Zi <me@ziyao.cc>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251209140006.54821-3-me@ziyao.cc