powerpc/mm: Fix section mismatch for mark_reserved_regions_for_nid

mark_reserved_regions_for_nid() is only called from do_init_bootmem(),
which is in .init.text, so it must be in the same section to avoid a
section mismatch warning.

Reported-by: Subrata Modak <subrata@linux.vnet.ibm.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>