dt-bindings: display: bridge: simple: document the Realtek RTD2171 DP-to-HDMI bridge

dt-bindings: display: bridge: simple: document the Realtek RTD2171 DP-to-HDMI bridge

The Realtek RTD2171 chipset is a transparent DisplayPort 1.4 to
HDMI 2.0 bridge.

This chipset is usually found in USB-C To HDMI Adapters and Docks,
or laptops to provide HDMI display output.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250908-topic-x1e80100-hdmi-v3-1-c53b0f2bc2fb@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>