media: staging: media: imx: imx7-media-csi: Import imx_media_dev_init() helper

To prepare for code refactoring, copy the imx_media_dev_init() helper
used by this driver from imx-media-dev-common.c. Rename the function to
avoid name clashes, and leave the v4l2_device notify handler out as it
only serves to implement control inheritance, which this driver doesn't
use. No functional change intended.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>