dt-bindings: usb: Add missing properties used in examples

With 'unevaluatedProperties' support implemented, the following warnings
are generated in the usb examples:

Documentation/devicetree/bindings/usb/intel,keembay-dwc3.example.dt.yaml: usb: usb@34000000: Unevaluated properties are not allowed ('reg' was unexpected)
Documentation/devicetree/bindings/usb/snps,dwc3.example.dt.yaml: usb@4a030000: Unevaluated properties are not allowed ('reg' was unexpected)

Add the missing property definitions.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ahmad Zainie <wan.ahmad.zainie.wan.mohamad@intel.com>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20211206174113.2295616-1-robh@kernel.org