IB/mlx5: Make create/destroy_ah available to userspace

Advertise that create_ah and destroy_ah verbs are accessible from
uverbs interface.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>