[media] DocBook: Document MEDIA_IOC_G_TOPOLOGY

Add description for this new media controller ioctl.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media/v4l/media-controller.xml

@@ -98,6 +98,7 @@
   &sub-media-func-ioctl;
   <!-- All ioctls go here. -->
   &sub-media-ioc-device-info;
+  &sub-media-ioc-g-topology;
   &sub-media-ioc-enum-entities;
   &sub-media-ioc-enum-links;
   &sub-media-ioc-setup-link;