This provides the necessary infrastructure for PowerMac specific drivers
(and actually some Open Firmware platform drivers on non-PowerMacs as
well provided somebody port them) to be properly probed & referenced via
the new driver model and be part of sysfs.

As-is, this patch doesn't break anything nor change any driver.  I'll
send you individual driver patches as I clean them up & get them tested
on as many machines as possible, though I don't expect much problems.