powerpc/numa: Fix build when CONFIG_NUMA=n

Add a non-NUMA definition for of_drconf_to_nid_single() to topology.h
so we have one even if powerpc/mm/numa.c is not compiled. On a
non-NUMA kernel the appropriate node id is always first_online_node.

Fixes: 72cdd117

 ("pseries/hotplug-memory: hot-add: skip redundant LMB lookup")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Scott Cheloha <cheloha@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201105223040.3612663-1-cheloha@linux.ibm.com