spapr_rtas: add get/set-power-level RTAS interfaces

spapr_rtas: add get/set-power-level RTAS interfaces

These interfaces manage the power domains that guest devices are
assigned to and are used to power on/off devices. Currently we
only utilize 1 power domain, the 'live-insertion' domain, which
automates power management of plugged/unplugged devices, essentially
making these calls no-ops, but the RTAS interfaces are still required
by guest hotplug code and PAPR+.

See docs/specs/ppc-spapr-hotplug.txt for a complete description of
these interfaces.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>