microblaze/pci: Move the remains of pci_32.c to pci-common.c

There's no point in keeping this separate. Even if microblaze grows
a 64-bit variant, it will probably be able to re-use that code as-is

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