Documentation: dt-bindings: add dt binding info for dwc2 fifo resizing

Tested-by: Robert Baldyga <[e-mail]>
Acked-by: Paul Zimmerman <[e-mail]>
Signed-off-by: Gregory Herrero <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

Documentation/devicetree/bindings/usb/dwc2.txt

···
 - dr_mode: shall be one of "host", "peripheral" and "otg"
   Refer to usb/generic.txt
 - g-use-dma: enable dma usage in gadget driver.
+- g-rx-fifo-size: size of rx fifo size in gadget mode.
+- g-np-tx-fifo-size: size of non-periodic tx fifo size in gadget mode.
+- g-tx-fifo-size: size of periodic tx fifo per endpoint (except ep0) in gadget mode.
 
 Example: