dt-bindings: display: bridge: Add waveshare DSI2DPI unit support

Add dt-binding documentation for waveshare DSI2DPI unit

Signed-off-by: Joseph Guo <qijian.guo@nxp.com>
Suggested-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250806-waveshare-v3-1-fd28e01f064f@nxp.com