PCI: layerscape: Factor out SCFG related function

For the LS1021a PCIe controller, some status registers are located in SCFG,
unlike other Layerscape devices.

Move SCFG-related code to ls1021_pcie_host_init() and rename
ls_pcie_link_up() to ls1021_pcie_link_up() because LTSSM status is also in
SCFG.

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