pci: layerscape: Manage PCIe EP compatible string via Kconfig

pci: layerscape: Manage PCIe EP compatible string via Kconfig

The ep node device tree name is governed by these bindings:
https://github.com/torvalds/linux/blob/master/Documentation/
devicetree/bindings/pci/layerscape-pci.txt#L24

As per above the ep compatible node contains platform name.
Therefore, define the ep node compatible as CONFIG to find the
pcie ep node in device tree during device tree fixup.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>