It is possible to adjust the fps rate by changing some register values.
This is function of the connected Xtal at the camera sensor, being a 27
MHz cristal needed, in order to support 640x480 at 30 fps.

For now, it will only calculate the values for fps. Later patches may
introduce V4L2 ioctls, to allow frequency rate adjustments.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>