mlxsw: reg: Add QoS Port DSCP to Priority Mapping Register

The QPDPM register controls the mapping from DSCP field to Switch
Priority for IP packets.

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