"Ball Hawk" is not an official attribute or ability in Madden NFL 15. It's community slang for a defensive back with elite interception skills—someone who consistently reads the quarterback and makes plays on the ball.
"Ball hawk" is subjective—build your roster to fit your defensive scheme (Cover 2, Cover 3, Man, etc.). High INT + PRC + ZCV is the classic formula for a user-controlled free safety who roams the deep middle.