This finishes refactoring by using the spapr_populate_memory_node helper
for all nodes and removing leftovers from spapr_populate_memory().

This is not a part of the previous patch because the patches look
nicer apart.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NAlexander Graf <agraf@suse.de>