riscv: define vmemmap before pfn_to_page calls

pfn_to_page & page_to_pfn depend on vmemmap being available before the calls
if kernel is configured with CONFIG_SPARSEMEM_VMEMMAP=y. This was caused
by NOMMU changes which moved vmemmap definition bellow functions definitions
calling pfn_to_page & page_to_pfn.

Noticed while compiled 5.5-rc2 kernel for Fedora/RISCV.

v2:
- Add a comment for vmemmap in source

Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Fixes: 6bd33e1e ("riscv: add nommu support")
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>