MFC 232403,232667:

• Add a bus_dma tag to each PCI bus that is a child of a Host-PCI bridge. The tag enforces a single restriction that all DMA transactions must not cross a 4GB boundary. Note that while this restriction technically only applies to PCI-express, this change applies it to all PCI devices as it is simpler to implement that way and errs on the side of caution.
• Add a softc structure for PCI bus devices to hold the bus_dma tag and a new pci_attach_common() routine that performs actions common to the attach phase of all PCI bus drivers. Right now this only consists of a bootverbose printf and the allocate of a bus_dma tag if necessary.
• Adjust all PCI bus drivers to allocate a PCI bus softc and to call pci_attach_common() from their attach routines.