[media] omap3isp: Add information on external subdev to struct isp_pipeline

Add pointer to external subdev, pixel rate of the external subdev and bpp of
the format to struct isp_pipeline.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>