media: v4l2-subdev: Store frame interval in subdev state

Subdev states store all standard pad configuration data, except for
frame intervals. Fix it by adding interval fields in the
v4l2_subdev_pad_config and v4l2_subdev_stream_config structures, with
corresponding accessor functions and a helper function to implement the
.get_frame_interval() operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>