[media] docs-rst: convert uAPI structs to C domain

instead of declaring the uAPI structs using usual refs, e. g.:
	.. _foo-struct:

Use the C domain way:
	.. c:type:: foo_struct

This way, the kAPI documentation can use cross-references to
point to the uAPI symbols.

That solves about ~100 undefined warnings like:
	WARNING: c:type reference target not found: foo_struct

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/vidioc-subdev-enum-frame-interval.rst

@@ -113,7 +113,7 @@ multiple pads of the same sub-device is not defined.
 
     -  .. row 6
 
-       -  struct :ref:`v4l2_fract <v4l2-fract>`
+       -  struct :c:type:`v4l2_fract`
 
        -  ``interval``