Document the link-frequencies property as recommended by the following
document:

https://www.kernel.org/doc/html/v6.9-rc1/driver-api/media/camera-sensor.html#handling-clocksSigned-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>