85xx/mpc8536ds: Use is_serdes_configured() to determine of PCIe enabled

The new is_serdes_configured covers a broader range of devices than the
PCI specific code.  Use it instead as we convert away from the
is_fsl_pci_cfg() code.

Additionally move to setting LAWs for PCI based on if its configured.
Also updated PCI FDT fixup code to remove PCI controllers from dtb if
they are configured.

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