dt-bindings: drm/bridge: Document Cadence DSI bridge bindings

Document the bindings used for the Cadence DSI bridge.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180421070846.10330-2-boris.brezillon@bootlin.com