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>