spapr: Allow dynamic creation of PHB

Now that we finally check for presence of dangling sysbus devices, make check
started complaining that the sPAPR PHB is one such device.

However, it really isn't. The spapr PHB is not really a traditional sysbus
device, but much more a special spapr pv device which is already able to get
created dynamically.

Move spapr to its own dynamic sysbus check handling and allow PHB devices to
get allocated dynamically.

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