media: ar0521: Add LINK_FREQ control

Add support for V4L2_CID_LINK_FREQ which currently reports a single
hard-coded frequency which depends on the fixed pixel clock.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>