sparc64: Fix non-SMP build.

Need to provide a dummy smp_fill_in_cpu_possible_map.

Fixes: 9b2f753e ("sparc64: Fix cpu_possible_mask if nr_cpus is set")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>