riscv: Remove unnecessary declaration

max_low_pfn and min_low_pfn are declared in linux/memblock.h,
and it also is included in arch/riscv/mm/init.c, drop unnecessary
declaration.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>