powerpc/pci: Merge ppc32 and ppc64 versions of phb_scan()

The two versions are doing almost exactly the same thing.  No need to
maintain them as separate files.  This patch also has the side effect
of making the PCI device tree scanning code available to 32 bit powerpc
machines, but no board ports actually make use of this feature at this
point.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>