powerpc: Fix warnings for arch/powerpc/mm/numa.c

Simple fixes for sparse warnings in this file.

Resolves:
arch/powerpc/mm/numa.c:198:24:
        warning: Using plain integer as NULL pointer

arch/powerpc/mm/numa.c:1157:5:
       warning: symbol 'hot_add_node_scn_to_nid' was not declared.
                Should it be static?

arch/powerpc/mm/numa.c:1238:28:
       warning: Using plain integer as NULL pointer

arch/powerpc/mm/numa.c:1538:6:
       warning: symbol 'topology_schedule_update' was not declared.
                Should it be static?

Signed-off-by: Robert C Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>