* 'Terrorist' designation would lead to punitive measures * Five Democratic senators urge Clinton to make the move (Adds more from State Department spokesman) By Sue Pleming WASHINGTON, May 11 (Reuters) - The U.S. State Department said on Tuesday it was looking into putting the Pakistani Taliban, the group tied to the failed car bombing in New York's Times Square, on the U.S. list of "foreign terrorist" groups. Adding the Pakistani Taliban to the list would trigger punitive measures such as freezing assets tied to the group, barring foreign nationals with links to it from entering the United States and...