drivers: media: imx477: Add V4L2_CID_LINK_FREQ control

Add V4L2_CID_LINK_FREQ as a read-only control with a value of 450 Mhz.
This will be used by the CFE driver to corretly setup the DPHY timing
parameters in the CSI-2 block.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>