[media] DT: export of_get_next_parent() for use by modules: fix modular V4L2

Currently modular V4L2 build with enabled OF is broken dur to the
of_get_next_parent() function being unavailable to modules. Export it to
fix the build.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>