PCI: dw-rockchip: Add endpoint mode support

PCI: dw-rockchip: Add endpoint mode support

The PCIe controller in rk3568 and rk3588 can operate in endpoint mode.

This endpoint mode support heavily leverages the existing code in
pcie-designware-ep.c.

Add support for endpoint mode to the existing pcie-dw-rockchip glue
driver.

[kwilczynski: squash with patch adding the PCI_ENDPOINT dependency]
Link: https://lore.kernel.org/linux-pci/20240607-rockchip-pcie-ep-v1-v5-10-0a042d6b0049@kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>