powernv has some code (derived from the spapr equivalent) used in device
tree generation which depends on the CPU's compatibility mode / logical
PVR.  However, compatibility modes don't make sense on powernv - at least
not as a property controlled by the host - because the guest in powernv
has full hypervisor level access to the virtual system, and so owns the
PCR (Processor Compatibility Register) which implements compatiblity modes.

Note: the new logic doesn't take into account kvmppc_smt_threads() like the
old version did.  However, if core->nr_threads exceeds kvmppc_smt_threads()
then things will already be broken and clamping the value in the device
tree isn't going to save us.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>