riscv: use NULL instead of a plain 0

sbi_remote_sfence_vma() & sbi_remote_fence_i() takes
a pointer as first argument but some macros call them with
a plain 0 which, while legal C, is frowned upon in the kernel.

Change this by replacing the 0 by NULL.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>