dt-bindings: usb: dwc2: add utmi optional clock

utmi clock is typically provided by PHY output. Add this optional clock,
as the core could use other clocks depending on the SoC where it's used.
This is needed on stm32mp15, when using the integrated full-speed PHY.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20230414084137.1050487-3-fabrice.gasnier@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>