dt-bindings: usb: qcom,dwc3: drop assigned-clocks

The binding does not have to specify assigned-clocks, because they are
already allowed by core DT schema.  On the other hand, fixed
assigned-clocks in the binding will not fit different boards or SoCs.
Exactly this is the case for Qualcomm SuperSpeed DWC3 USB SoC controller
binding, where few boards have different assigned-clocks:

  ipq8074-hk10-c1.dtb: usb@8cf8800: assigned-clocks: [[5, 131], [5, 132], [5, 133]] is too long
  sdm660-xiaomi-lavender.dtb: usb@a8f8800: assigned-clocks: [[37, 92], [37, 91], [38, 64]] is too long

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230723141550.90223-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>