powerpc/mm: Fix section mismatch for read_n_cells

read_n_cells() cannot be marked as .devinit.text since it is referenced
from two functions that are not in that section: of_get_lmb_size() and
hot_add_drconf_scn_to_nid().

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>