spapr: fix potential memory leak in spapr_core_plug()

Since commit 5c1da81215c7 ("spapr: Remove unnecessary differences between
hotplug and coldplug paths"), the CPU DT for the DRC is always allocated.
This causes a memory leak for pseries-2.6 and older machine types, that
don't support CPU hotplug and don't allocate DRCs for CPUs.

Reported-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
