mlxsw: reg: Add Tunneling NVE Underlay Multicast Table Register

This register builds the linked list of underlay destination IPs used
for BUM traffic on the overlay.

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