pcie_ep: layerscape: Add the multiple function support

Add the multiple function support for Layerscape platform, some PEXs
of Layerscaple platform have more than one PF.

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>