spapr_drc: add unrealize method to physical DRC class

When hot-unplugging a PHB, all its PCI DRC connectors get unrealized. This
patch adds an unrealize method to the physical DRC class, in order to undo
registrations performed in realize_physical().

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
