[media] media: Links setup

Create the following ioctl and implement it at the media device level to
setup links.

- MEDIA_IOC_SETUP_LINK: Modify the properties of a given link

The only property that can currently be modified is the ENABLED link
flag to enable/disable a link. Links marked with the IMMUTABLE link flag
can not be enabled or disabled.

Enabling or disabling a link has effects on entities' use count. Those
changes are automatically propagated through the graph.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Stanimir Varbanov <svarbanov@mm-sol.com>
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/Documentation/DocBook/v4l/media-controller.xml b/Documentation/DocBook/v4l/media-controller.xml
index 2c4fd2b..2dc25e1 100644
--- a/Documentation/DocBook/v4l/media-controller.xml
+++ b/Documentation/DocBook/v4l/media-controller.xml
@@ -85,4 +85,5 @@
   &sub-media-ioc-device-info;
   &sub-media-ioc-enum-entities;
   &sub-media-ioc-enum-links;
+  &sub-media-ioc-setup-link;
 </appendix>