Add a device tree node representing PCIe endpoint mode.

The controller can either be configured to run in Root Complex or Endpoint
mode.

If a user wants to run the controller in endpoint mode, the user has to
disable the pcie3x4 node and enable the pcie3x4_ep node.
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20240607-rockchip-pcie-ep-v1-v5-12-0a042d6b0049@kernel.orgSigned-off-by: Heiko Stuebner <heiko@sntech.de>