[media] media: add support to link interfaces and entities

Now that we have a new graph object called "interfaces", we
need to be able to link them to the entities.

Add a linked list to the interfaces to allow them to be
linked to the entities.

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