powerpc/fsl_pci: Fix device tree fixups for newer platforms

We assumed that only a small set of compatiable strings would be needed
to find the PCIe device tree nodes to be fixed up.  However on newer
platforms the simple rules no longer work.  We need to allow specifying
the PCIe compatiable string for each individual SoC.

We introduce CONFIG_SYS_FSL_PCIE_COMPAT for this purpose and set it if
the default isn't sufficient.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>