[media] v4l2-mc: add a generic function to create the media graph

The em28xx_v4l2_create_media_graph() is almost generic enough to
be at the core, as an ancillary function. Make it even more generic,
by getting rid of em28xx-specific code, relying only at the
media_device, in order to discover all entities found on PC-customer's
hardware and add it at the V4L2 core.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>