[media] omap3isp: Support configurable HS/VS polarities

Add two fields to the ISP parallel platform data to set the HS and VS
signals polarities.

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