V4L/DVB (9974): v4l2-dev: allow drivers to pass v4l2_device as parent

Drivers that use v4l2_device can set that as parent pointer in the v4l2_dev
field instead of using the struct device parent field.

This allows v4l2-dev.c to check whether this driver is v4l2_device based,
and if so then it can offer additional services.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>