spapr: Move creation of ibm,dynamic-reconfiguration-memory dt node

Currently this node with information about hotpluggable memory is created
from spapr_dt_cas_updates().  But that's just a hangover from when we
created it only as a diff to the device tree at CAS time.  Now that we
fully rebuild the DT as CAS time, it makes more sense to create this along
with the rest of the memory information in the device tree.

So, move it to spapr_populate_memory().  The patch is huge, but it's nearly
all just code motion.

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