IB/mlx5: Add null_mkey access

Add mlx5_cmd_null_mkey() function to access null_mkey information
from firmware.

Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>