dt-bindings: usb: dwc2: add iommus

The DWC2 node might use IOMMU (e.g. Altera Stratix10), so add "iommus"
property.  This fixes warnings like:

  arch/arm64/boot/dts/altera/socfpga_stratix10_socdk.dt.yaml:
    usb@ffb40000: 'iommus' does not match any of the regexes: 'pinctrl-[0-9]+'

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