mlxsw: reg: Add Tunneling NVE Decapsulation ECN Mapping Register

This register configures the actions that are done during NVE
decapsulation based on the ECN bits.

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