media: ov5645: Add control to export pixel clock value

Add suport for standart read-only 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 value is
updated when the sensor mode is changed - on set_format.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>