mlxsw: reg: Add Tunneling NVE Encapsulation ECN Mapping Register

This register performs mapping from overlay ECN to underlay ECN during
NVE encapsulation.

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