Add support for offloading mqprio qdisc to sparx5 switch.

The offloaded mqprio qdisc currently does nothing by itself, but serves
as an attachment point for other qdiscs (tbf, ets etc.)
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>