media: dt-bindings: imx7-csi: Document a single CSI clock

As per the i.MX7D Reference Manual only the MCLK is used for
the CSI block, so only document this single clock.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>