Add PCI_ID_OFW_IOMMU to the pci ecam ACPI driver

Teach the pci host generic ACPI attachment about PCI_ID_OFW_IOMMU. This
will be used by the arm64 smmu IOMMU driver to read the xref and ID
this interface provides in a bus-agnostic way.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D39182