media: rkisp1: Rename rkisp1_subdev_notifier() to rkisp1_subdev_notifier_register()

The function name isn't very clear, rename it to
rkisp1_subdev_notifier_register().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>