mlxsw: core: Add QOS_ACTION

The QOS_ACTION is used for manipulating the QoS attributes of a packet.
Add the corresponding defines and helpers, in particular for the
switch_priority override.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>