doc: Add "ti,am437x-dwc3" comaptible for dwc3 glue

Add the compatible "ti,am437x-dwc3" for dwc3 glue driver.
Signed-off-by: George Cherian <george.cherian@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

Documentation/devicetree/bindings/usb/omap-usb.txt

@@ -44,7 +44,9 @@ Board specific device node entry
 };
 
 OMAP DWC3 GLUE
- - compatible : Should be "ti,dwc3"
+ - compatible : Should be
+	* "ti,dwc3" for OMAP5 and DRA7
+	* "ti,am437x-dwc3" for AM437x
  - ti,hwmods : Should be "usb_otg_ss"
  - reg : Address and length of the register set for the device.
  - interrupts : The irq number of this device that is used to interrupt the