Recent changes introduced cannot_instantiate_with_device_add_yet
and removed capability of adding yet another PCI host bridge via
command line for SPAPR platform (POWERPC64 server).

This brings the capability back and puts SPAPR PHB into "bridge"
category.

This is not much use for emulated PHB but it is absolutely required
for VFIO as we put an IOMMU group onto a separate PHB on SPAPR.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NAlexander Graf <agraf@suse.de>