PCI: layerscape: Update ls_add_pcie_port()

Update the ls_add_pcie_port() signature to keep it consistent with the
other DesignWare-based host drivers.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>