[POWERPC] Removed remnants of bus_offset

Removed the remants of bus_offset and use self_busno in the mv64x60 case
and use pci_assign_all_buses on 83xx/85xx.

83xx/85xx have multiple PHBs and the firmwares on these devices tend not
to handle topologies with P2P bridges well so we let Linux just reassign
the bus numbers to match.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>