[POWERPC] Use global_number in ppc32 pci_controller

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: Kumar Gala <galak@kernel.crashing.org>