mlxsw: reg: Add Management Temperature Warning Event Register

Add MTWE (Management Temperature Warning Event) register, which is used
for over temperature warning.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>