mlxsw: Implement flash update status notifications

Implement mlxfw status_notify op by passing notification down to
devlink. Also notify about flash update begin and end.

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