[media] omap3isp: Separate external link creation from platform data parsing

Move the code which connects the external entity to an ISP entity into a
separate function. This disconnects it from parsing the platform data.

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