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>