dt-bindings: usb: keystone-usb: Add ti,am654-dwc3 support

The AM654 SoC from TI contains a DWC3 controller. Add
support for it.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>