drm/vc4: Add RST to bring together vc4 kerneldoc.

This doesn't yet produce coherent documentation of the module, but at
least gets the kerneldoc built and somewhat glued together.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227201144.10970-3-eric@anholt.net

Documentation/gpu/index.rst

@@ -12,6 +12,7 @@ Linux GPU Driver Developer's Guide
    drm-uapi
    i915
    tinydrm
+   vc4
    vga-switcheroo
    vgaarbiter
    todo