dt-bindings: Add "external-facing" PCIe port property

Provide a way for the firmware to tell the OS which devices are external to
the machine and therefore untrusted.  The property can describe for example
Thunderbolt and other user-accessible ports, which should always have the
strongest IOMMU protection.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Grant Likely <grant.likely@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>