dt-bindings: remove file that was added accidentally

I think this snuck in when I applied the patch for f97decac (didn't
apply cleanly, required some manual applying + git-add).  It is unused
and shouldn't be here.  My bad.

Fixes: f97decac "drm/msm: Support multiple ringbuffers"
Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>