dt-bindings: phy: Add Rockchip HDMI/eDP Combo PHY schema

Add dt-binding schema for the HDMI/eDP Transmitter Combo PHY found on
Rockchip RK3588 SoC.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20240214-phy-hdptx-v4-1-e7974f46c1a7@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>