riscv: Fix undefined reference to vmemmap_populate_basepages

Using CONFIG_SPARSEMEM_VMEMMAP instead of CONFIG_SPARSEMEM to fix
following build issue.

  riscv64-linux-ld: arch/riscv/mm/init.o: in function 'vmemmap_populate':
  init.c:(.meminit.text+0x8): undefined reference to 'vmemmap_populate_basepages'

Cc: Logan Gunthorpe <logang@deltatee.com>
Fixes: d95f1a54 ("RISC-V: Implement sparsemem")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>