PCI: artpec: Add Axis ARTPEC-6 PCIe controller driver

The Axis ARTPEC-6 SoC integrates a PCIe controller from Synopsys.  Add a
new driver that provides the small glue needed to use the existing
DesignWare driver to make it work on the Axis ARTPEC-6 SoC.

[bhelgaas: return errors directly without gotos, fold in section mismatch
fix]
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>