drivers/gpu/pvr/pvr_governor: add a simple PVR GPU governor

this simple PVR GPU governor will scale between 3 speeds:
  idle -- GPU not active
  nominal -- GPU active with low graphical load
  turbo -- GPU active and boosted to max frequency

This should further save battery by intelligently selecting GPU frequency
based on GPU loading.

Signed-off-by: Paul Reioux <reioux@gmail.com>