[media] v4l: vsp1: Store active selection rectangles in a pad config structure

Use the pad config structure part of the vsp1_entity to store all active
pad selection rectangles. This generalizes the code to operate on pad
config structures.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>