Fix gpiobus to return BUS_PROBE_GENERIC insted of BUS_PROBE_SPECIFIC (0) so
it can be overriden by its OFW/FDT version.

Give a chance for GPIO devices that implement the device_identify method to
attach.

Approved by:	adrian (mentor)
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.

MFC after:	3 days
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.