The market rose in the run up to Nvidia's NVDA.O second-quarter results Wednesday. By Caroline Valetkevitch NEW YORK, Aug 25 (Reuters) - U.S. stocks ended a volatile session higher on Friday as investors digested comments from Federal Reserve Chair Jerome Powell that the U.S. central bank may need to raise interest rates further to ensure inflation is contained. But he is still holding on tightly to this notion that they are watching it carefully and they still have work to do," said Michael Arone, chief investment strategist at State Street Global Advisors in Boston.