MFC: r287726

• Factor out the common and generic parts of the sparc64 host-PCI-bridge drivers into the revived sys/sparc64/pci/ofw_pci.c, previously already serving a similar purpose. This has been done with sun4v in mind, which explains a) the otherwise not that obvious scheme employed and b) why reusing sys/powerpc/ofw/ofw_pci.c was even lesser an option.
• Add a workaround for QEMU once again not emulating real machines, in this case by not providing the OFW_PCI_CS_MEM64 range. [1]

Submitted by: jhb [1]