[media] media: add support for decoder as one of media entity types

A lot of SOCs including Texas Instruments Davinci family mainly use
video decoders as input devices. This patch adds a flag
'MEDIA_ENT_T_V4L2_SUBDEV_DECODER' media entity type for decoder's.
Along side updates the documentation for this media entity type.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>