media: imx: add 8-bit grayscale support

The IPUv3 code has 8-bit grayscale capture support.
Enable imx-media to use it.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>