[media] media i.MX27 camera: add start_stream and stop_stream callbacks

Add "start_stream" and "stop_stream" callback in order to enable
and disable the eMMa-PrP properly and save CPU usage avoiding
IRQs when the device is not streaming. This also makes the driver
return 0 as the sequence number of the first frame.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
[g.liakhovetski@gmx.de: remove the mx27_camera_emma() macro]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>