dt-bindings: usb: clean up compatible property

Add quotation marks around the compatible string to avoid ambiguity due
to following punctuation, and define the VID and PID components.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -5,11 +5,11 @@ The reference binding doc is from:
 http://www.devicetree.org/open-firmware/bindings/usb/usb-1_0.ps
 
 Required properties:
-- compatible: usbVID,PID. The textual representation of VID, PID shall
-  be in lower case hexadecimal with leading zeroes suppressed. The
-  other compatible strings from the above standard binding could also
-  be used, but a device adhering to this binding may leave out all except
-  for usbVID,PID.
+- compatible: "usbVID,PID", where VID is the vendor id and PID the product id.
+  The textual representation of VID and PID shall be in lower case hexadecimal
+  with leading zeroes suppressed. The other compatible strings from the above
+  standard binding could also be used, but a device adhering to this binding
+  may leave out all except for "usbVID,PID".
 - reg: the port number which this device is connecting to, the range
   is 1-255.