ppc/pnv: add a "ibm,opal/power-mgt" device tree node on POWER9

Activate only stop0 and stop1 levels. We should not need more levels
when under QEMU.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190307223548.20516-15-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>