The DWC2 USB bindings require clock-names property, to provide such to
fix warnings like:

  socfpga_stratix10_swvp.dtb: usb@ffb40000: 'clock-names' is a required property
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>