dt-bindings: Document the VC4 DSI module nodes.

These are part of the vc4 display pipeline.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161214194621.16499-7-eric@anholt.net