target/ppc: Add stub implementation of the PSSCR

The PSSCR register added in POWER9 controls certain power saving mode
behaviours.  Mostly, it's not relevant to TCG, however because qemu
doesn't know about it yet, it doesn't synchronize the state with KVM,
and thus it doesn't get migrated.

To fix that, this adds a minimal stub implementation of the register.
This isn't complete, even to the extent that an implementation is
possible in TCG, just enough to get migration working.  We need to
come back later and at least properly filter the various fields in the
register based on privilege level.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
