media: imx: imx6-media-csi: Support clamping Y10 and Y12 to Y8

10-bit and 12-bit greyscale input data to the CSI can be written as
8-bit data to memory. Support this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>