[media] media-device: add pads and links to media_device

The MC next gen API sends objects to userspace grouped by
their types.

In the case of pads and links, in order to improve performance
and have a simpler code, the best is to store them also on
separate linked lists at MC.

If we don't do that, we would need this kind of interaction
to send data to userspace (code is in structured english):

	for each entity:
		for each pad:
			store pads

	for each entity:
		for each link:
			store link

	for each interface:
		for each link:
			store link

With would require one nested loop for pads and two nested
loops for links. By using  separate linked lists for them,
just one loop would be enough.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>