arm64: dts: agilex/stratix10: add clock-names to USB DWC2 node

USB DWC2 requires clock-names:

  arm64/boot/dts/altera/socfpga_stratix10_socdk_nand.dt.yaml:
    usb@ffb00000: 'clock-names' is a required property

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>