media: staging: media: imx: imx7-media-csi: Import notifier helpers

To prepare for code refactoring, copy the V4L2 async notifier helper
code used by this driver verbatim from imx-media-dev-common.c. Rename
some functions to avoid name clashes. No functional change included.

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>