target/ppc: Implement TIDR

target/ppc: Implement TIDR

This adds a trivial implementation of the TIDR register added in
POWER9.  This isn't particularly important to qemu directly - it's
used by accelerator modules that we don't emulate.

However, since qemu isn't aware of it, its state is not synchronized
with KVM and therefore not migrated, which can be a problem.

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>