powerpc/pci: Make pcibios_allocate_bus_resources more robust

To properly fix PCI hotplug, it's useful to be able to make the fixup
passes on all devices whether they were just hot plugged or already
there.

However, pcibios_allocate_bus_resources() wouldn't cope well with
being called twice for a given bus.  This makes it ignore resources
that have already been allocated, along with adding a bit of debug
output.

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