MFC: r258051, r258052

MFC:	r258051, r258052

r258052:
Following the approach with ACPI DMAR on x86, split IOMMU handling into
a variant PCI bus instead of trying to shoehorn it into the PCI host bridge
adapter. Besides matching better the architecture on other platforms, this
also allows systems with multiple partitionable endpoints per PCI host
bridge to work correctly.

r258051:
Actually add IOMMU domain to the list of known mappings. This fixes a bug
where multiple devices in the same IOMMU domain would be allocated
conflicting mappings unless they also shared a DMA tag.

git-svn-id: svn://svn.freebsd.org/base/stable/10@259257 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f