riscv: mm: init: make pt_ops_set_[early|late|fixmap] static

These three functions are only used in init.c, so make them static.
Fix W=1 warnings like below:

arch/riscv/mm/init.c:721:13: warning: no previous prototype for function
'pt_ops_set_early' [-Wmissing-prototypes]
   void __init pt_ops_set_early(void)
               ^

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20220516143204.2603-1-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>