dt-bindings: PCI: Add CIX Sky1 PCIe Root Complex bindings

dt-bindings: PCI: Add CIX Sky1 PCIe Root Complex bindings

Document the bindings for CIX Sky1 PCIe Controller configured in Root
Complex mode with five Root Ports.

The controller supports 4 INTx, MSI and MSI-X interrupts with the help of
the ARM GICv3 interrupt controller.

Signed-off-by: Hans Zhang <hans.zhang@cixtech.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20251108140305.1120117-6-hans.zhang@cixtech.com