powerpc/pci: Use common PHB resource hookup

The 32-bit and 64-bit powerpc PCI code used to set up the resource
pointers of the root bus of a given PHB in completely different
places.

This unifies this in large part, by making 32-bit use a routine very
similar to what 64-bit does when initially scanning the PCI busses.

The actual setup of the PHB resources itself is then moved to a
common function in pci-common.c.

This should cause no functional change on 64-bit.  On 32-bit, the
effect is that the PHB resources are going to be setup a bit earlier,
instead of being setup from pcibios_fixup_bus().

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>