media: ov5645: Add control to export CSI2 link frequency

Add suport for standard integer menu V4L2_CID_LINK_FREQ control.
The CSI2 link frequency value is specific for each sensor mode so the
sensor mode structure is extended to add this. The control is made
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>