Support intel-pstate driver.

Hi, the extension stopped working on my  system since I've updated to linux-3.10. This is due to the use of the new intel P-state driver. This patch makes the extension work again. 

Intel P-state driver was introduced in linux-3.9 (default in linux-3.10 I
think) and doesn't support directly frequencies. It allows only to switch
between the two available governors: performance and powersave. Show the
governor name instead of the frequency.