media: add digital video decoder entity functions

Add a new media entity function definition for digital TV decoders:
MEDIA_ENT_F_DTV_DECODER

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>