libcamera: pipeline: raspberrypi: Move StaggeredCtrl to libcamera namespace

The StaggeredCtrl class, part of the Raspberry Pi pipeline handler, is
part of libcamera. Move it to the libcamera namespace to simplify usage
of libcamera APIs.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>