dt-bindings: phy: imx8mq-usb: add phy tuning properties

Add optional properties for tuning of usb phy.

Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230516-lustige-usb-phy-dinge-v2-1-3383a0de34ac@pengutronix.de


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