media: dt-bindings: media: hynix,hi846: add link-frequencies description

link-frequencies is required but only mentioned in the example. Add
it to the description.

Fixes: f3ce7200 ("media: dt-bindings: media: document SK Hynix Hi-846 MIPI CSI-2 8M pixel sensor")
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>