dt-bindings: phy: Add bindings doc for Sunplus USB2 PHY driver

Add bindings doc for Sunplus USB2 PHY driver

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Vincent Shih <vincent.sunplus@gmail.com>
Link: https://lore.kernel.org/r/1658717052-26142-3-git-send-email-vincent.sunplus@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>