PCI: imx6: Add i.MX PCIe EP mode support

i.MX PCIe is one dual mode PCIe controller.

Add i.MX PCIe EP mode support here, and split the PCIe modes to the Root
Complex mode and Endpoint mode.

Link: https://lore.kernel.org/r/1673847684-31893-12-git-send-email-hongxing.zhu@nxp.com


Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>