mlxsw: reg: Add Monitoring FW Debug Register

Introduce MFDE register that is passed through MFDE trap in case of
fatal FW event.

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