PCI: OF: Support "external-facing" property

Set the "untrusted" attribute to any PCIe port that has an
"external-facing" device tree property.  Any device downstream of this port
will inherit the attribute and have only the strictest IOMMU protection.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>