[media] media: Add KernelDoc documentation for struct media_entity_graph

KernelDoc doesn't appear to handle anonymous structs defined inside
another gracefully. As the struct is internal to the framework graph walk
algorithm, detailed documentation isn't seen very important.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>