dt-bindings: usb: Add Realtek DHC RTD SoC Type-C

Document the device-tree bindings for Realtek SoCs Type-C.
Realtek DHC (digital home center) RTD SoCs support a Type-C module.

Link: https://lore.kernel.org/lkml/20230904051253.23208-2-stanley_chang@realtek.com/
Signed-off-by: Stanley Chang <stanley_chang@realtek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>