[media] v4l: vsp1: Store pipeline pointer in rwpf

This prepares for dynamic pipeline allocation by providing a field that
can be used to store the pipeline pointer atomically under driver
control.

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