PCI: layerscape: Factor out ls_pcie_establish_link()

All other DesignWare-based drivers have a *_establish_link() function.

This functionality is trivial for Layerscape, but factor out a
ls_pcie_establish_link() for consistency with the other drivers.  No
functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>