PCI: artpec6: Remove unnecessary artpec6_pcie_link_up()

Remove artpec6_pcie_link_up(); the generic dw_pcie_link_up() does the same
thing, so we don't need a device-specific version.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>