Moved the low hanging fruit that was either identical or close
to it between ppc32 & ppc64 for PCI into pci-common.c
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>

Make the pci_controller struct use global_number for the PHB domain number
instead of index to match what ppc64 does and reuse its pci_domain_nr code.

Introduced a pci-common.c to handle shared code between ppc32 & ppc64.
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>