media: ov5645: Add control to export pixel clock frequency

Add suport for standard V4L2_CID_PIXEL_RATE control. The pixel clock
frequency value is specific for each sensor mode so the sensor mode
structure is extended to add this. The control is read-only and its
value is updated when the sensor mode is changed - on set_format.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>