powerpc/32: Restore previous version of 32-bit PCI code

When I removed the powermac support from arch/ppc/kernel/pci.c,
I overlooked the fact that that file is used in 32-bit ARCH=powerpc
builds.  To prevent problems in future, restore the original version
of that file as arch/powerpc/kernel/pci_32.c, and use that.

Signed-off-by: Paul Mackerras <paulus@samba.org>