AMSC, a Devens firm that makes control systems for wind turbines, said Tuesday that it has filed an appeal with China’s Supreme People’s Court related to dismissals by lower courts of AMSC’s civil case against Sinovel Wind Group Co. A wind turbine company, Sinovel was AMSC’s largest customer until it stopped accepting shipments of AMSC products. In multiple legal actions, AMSC is now suing Sinovel and seeking more than $1.2 billion in damages and payments.