powerpc/pci: Move pseries code into pseries platform specific area

There doesn't appear to be any specific reason that we need to setup the
pseries specific notifier in generic arch pci code.  Move it into pseries
land.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>