drm/vc4: Recognize a more specific compatible string for V3D.

The Raspberry Pi Foundation's firmware updates are shipping device
trees using the old string, so we'll keep recognizing that as this rev
of V3D.  Still, we should use a more specific name in the upstream DT
to clarify which board is being supported, in case we do other revs of
V3D in the future.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>