[media] DocBook: handle enums on frontend.h

In order to be sure that all enum definitions will be documented,
let's parse the enum values and add xref links to them.

Lots of missing references will be risen as we miss adding
id's to those symbols at the documentation. Next patches will
fix this.

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