RISC-V: Make NUMA depend on SMP

In theory these are orthogonal, but in practice all NUMA systems are
SMP.  NUMA && !SMP doesn't build, everyone else is coupling them, and I
don't really see any value in supporting that configuration.

Fixes: 4f0e8eef

 ("riscv: Add numa support for riscv64 platform")
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Atish Patra <atishp@atishpatra.org>
Reported-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by:  Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>