[media] tvp5150: add internal signal generator to HW input list

Some tvp5150 variants, have an internal generator that can generate a
black screen output. Since this is a HW block, it should be in the HW
inputs list.

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