media: i2c: imx219: Implement V4L2_CID_LINK_FREQ control

This control is needed for imx219 driver, as the link frequency
is independent from the pixel rate in this case, and can't be
calculated from the pixel rate.

Signed-off-by: Andrey Konovalov <andrey.konovalov@linaro.org>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>