MFC:	r258722, r258757

r258722:
Give some output about the CPU clock on IBMPOWER machines, currently read
from OF. Linux does it similar, means they also read the OF values and
display them.
r258757:
Use the Open Firmware-based CPU frequency determination as a generic
fallback if we can't measure CPU frequency. This is also useful on a
variety of embedded systems using FDT.
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.