PCI: layerscape: Add CONFIG_PCI_LAYERSCAPE_EP to build EP/RC separately

Add CONFIG_PCI_LAYERSCAPE_EP so that endpoint and host controller
drivers can be built separately.

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>