riscv: Fix spelling mistake "SPARSEMEM" to "SPARSMEM"

There is a spelling mistake when SPARSEMEM Kconfig copy.

Fixes: a5406a7f

 ("riscv: Correct SPARSEMEM configuration")
Cc: stable@vger.kernel.org
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>