PCI: rcar-gen4: Add endpoint mode support

Add R-Car Gen4 PCIe controller for endpoint mode. This controller is based
on Synopsys DesignWare PCIe.

Link: https://lore.kernel.org/linux-pci/20231018085631.1121289-14-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>