ARM: at91/DT: atmel_usba correct atmel,vbus-gpio meaning

atmel,at91sam9rl-udc is a USB gadget, it has no means to control vbus.
atmel,vbus-gpio is in fact used to detect the presence of vbus.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: modify the commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

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

@@ -57,8 +57,8 @@ Required properties:
  - ep childnode: To specify the number of endpoints and their properties.
 
 Optional properties:
- - atmel,vbus-gpio: If present, specifies a gpio that needs to be
-   activated for the bus to be powered.
+ - atmel,vbus-gpio: If present, specifies a gpio that allows to detect whether
+   vbus is present (USB is connected).
 
 Required child node properties:
  - name: Name of the endpoint.