mlxsw: core: Add DSCP, ECN, dscp_rw to QOS_ACTION

The QOS_ACTION is used for manipulating the QOS attributes of the packet.
Add the defines and helpers related to DSCP and ECN fields, and dscp_rw.

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>