PCI: layerscape: Add EP mode support

Add the PCIe EP mode support to the layerscape platform controller.

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Minghuan Lian <minghuan.lian@nxp.com>
Reviewed-by: Zhiqiang Hou <zhiqiang.hou@nxp.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>