microblaze/pci: Remove powermac originated cruft

The whole business with re-assigning all bus numbers, creating
an OF bus "map" etc... is ancient powermac stuff that you really
don't care about on microblaze.

Similarly pci_device_from_OF_node() is unused and by getting rid
of it we can get rid of a whole lot of code otherwise unused on
this architecture

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Michal Simek <monstr@monstr.eu>