media: imx7-media-csi: Acquire a single clock

As per the i.MX7D Reference Manual only the MCLK is used for
the CSI block, so only handle 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>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>