Combine first 5 ingredients in a large saucepan; bring to a boil. Cover, reduce heat, and simmer 10 minutes. Partially mash tomatoes and beans with a potato masher. Stir in chicken; cook 2 minutes or until thoroughly heated. Remove from heat; stir in oil. Place 1 1/4 cups soup in each of four bowls. Top each serving with 2 tablespoons sour cream and 1 tablespoon cilantro.