Preheat gas grill to medium heat or preheat grill pan. Combine the first 4 ingredients in a small bowl, reserve for use after potatoes are cooked. Combine oil and sweet potatoes in a medium bowl; toss to coat. (If preparing in advance, put coated sweet potatoes in a container/bag and put in the refrigerator. If possible, take this out of the refrigerator 30 minutes prior to cooking time to bring to room temp.) Coat cooking surface with cooking spray. Add potatoes, and grill/cook for 10 minutes (if at room temp and a little longer if directly from the refrigerator), turning occasionally. Place potatoes in a large bowl; sprinkle with cumin mixture and cilantro. Toss gently.