media: imx: imx7_mipi_csis: Add greyscale formats support

Add support for the 8-, 10- and 12-bit greyscale media bus formats, and
map them to the CSI-2 RAW8, RAW10 and RAW12 formats respectively.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.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+huawei@kernel.org>