ppc/pnv: Introduce PnvMachineClass::dt_power_mgt()

ppc/pnv: Introduce PnvMachineClass::dt_power_mgt()

We add an extra node to advertise power management on some machines,
namely powernv9 and powernv10. This is achieved by using the
pnv_is_power9() and pnv_is_power10() helpers.

This can be achieved with QOM. Add a method to the base class for
powernv machines and have it implemented by machine types that
support power management instead.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <157623839642.360005.9243510140436689941.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>