dt-bindings: usb: keystone-dwc3: Validate DWC3 sub-node

TI Keystone DWC3 compatible DT node is supposed to have a DWC USB3
compatible sub-node to describe a fully functioning USB interface.
Since DWC USB3 has now got a DT schema describing its DT node, let's make
sure the TI Keystone DWC3 sub-node passes validation against it.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201210090944.16283-18-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>