PPC: Use default pci bus name for grackle and heathrow

There's no good reason to call our bus "pci" rather than let the default
bus name take over ("pci.0").

The big downside to calling it different from anyone else is that tools
that pass -device get confused. They are looking for a bus "pci.0" rather
than "pci".

To make life easier for everyone, let's just drop the name override.

Signed-off-by: Alexander Graf <agraf@suse.de>
