To improve OPAL/skiboot support. We don't need to strictly model these
XSCOM accesses.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190307223548.20516-14-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>