media: imx: imx7-mipi-csis: Implement the .enum_mbus_code() operation

Implement the subdev pad .enum_mbus_code() operation to enumerate media
bus codes on the sink and source pads.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-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>