drivers: media: imx519: Add V4L2_CID_LINK_FREQ control

Add V4L2_CID_LINK_FREQ as a read-only control with a value of 408 Mhz.
This will be used by the CFE driver to corretly setup the DPHY timing
parameters in the CSI-2 block.

Signed-off-by: Lee Jackson <lee.jackson@arducam.com>