media: imx: imx7-media-csi: Reorganize code in sections

To prepare for a major rework of the hardware initialization, and make
the code easier to read, reorder functions to group them in sections. No
functional change is included.

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>