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