drm/v3d: Add missing v3d documentation structure.

This was a failure of "git add" on my part -- we already referenced
the doc from drivers.rst.

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703170515.6298-3-eric@anholt.net
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>