[media] tw686x: Add support for DMA scatter-gather mode

Now that the driver has the infrastructure to support more
DMA modes, let's add the DMA scatter-gather mode.

In this mode, the device delivers sequential top-bottom
frames. The scatter-gather logic is based on staging's
tw686x-kh driver (by Krzysztof Ha?asa).

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