mlxsw: reg: Add Tunneling NVE QoS Default Register

The TNQDR register configures the default QoS settings for NVE
encapsulation.

It will be used to set the default DSCP of each port to 0, so that when
DSCP is set to inherit and the overlay packet does not have an IP header
the outer DSCP will be set to 0, in accordance with the software data
path.

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