Documentation: dt-bindings: add dt binding info for dwc2 g-use-dma

Indicate if gadget driver must use dma for usb transfers.
Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/dwc2.txt

@@ -20,6 +20,7 @@ Optional properties:
 Refer to phy/phy-bindings.txt for generic phy consumer properties
 - dr_mode: shall be one of "host", "peripheral" and "otg"
   Refer to usb/generic.txt
+- g-use-dma: enable dma usage in gadget driver.
 
 Example: