powerpc/mm: Export memory_add_physaddr_to_nid() for modules

The cxl_pmem module wants to call memory_add_physaddr_to_nid(), so
export the symbol.

Link: http://lore.kernel.org/r/87sfmkbfyg.fsf@mpe.ellerman.id.au
Fixes: 04ad63f0 ("cxl/region: Introduce cxl_pmem_region objects")
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

arch/powerpc/mm/mem.c

@@ -54,6 +54,7 @@ int memory_add_physaddr_to_nid(u64 start)
 {
 	return hot_add_scn_to_nid(start);
 }
+EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
 #endif
 
 int __weak create_section_mapping(unsigned long start, unsigned long end,