V4L/DVB (10687): v4l2-common/v4l2-spec: support/document write-only and button controls

The controls V4L2_CID_PAN_RESET and V4L2_CID_TILT_RESET changed their type
to button controls (these are unused at the moment, so this is a safe change).

The controls V4L2_CID_PAN_RELATIVE, V4L2_CID_TILT_RELATIVE, V4L2_CID_FOCUS_RELATIVE
and V4L2_CID_ZOOM_RELATIVE are marked as write-only controls.

Cc: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>