spapr: populate PHB DRC entries for root DT node

This add entries to the root OF node to advertise our PHBs as being
DR-capable in accordance with PAPR specification.

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: Greg Kurz <groug@kaod.org>
Message-Id: <155059670897.1466090.10843921337591637414.stgit@bahia.lab.toulouse-stg.fr.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>