PCI: imx6: Rename imx6_pcie_start_link() to imx6_pcie_establish_link()

Rename imx6_pcie_start_link() to imx6_pcie_establish_link() to follow the
convention of other DesignWare-based host drivers.  No functional change.

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