Document DT bindings for PCIe Endpoint controller found in Rockchip SoCs.

Link: https://lore.kernel.org/linux-pci/20240607-rockchip-pcie-ep-v1-v5-6-0a042d6b0049@kernel.orgSigned-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>