RDMA/mlx5: Move struct mlx5_core_mkey to mlx5_ib

Move mlx5_core_mkey struct to mlx5_ib, as the mlx5_core doesn't use it
at this point.

Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>