of: Add vendor prefix for ON Semiconductor Corp.

The used vendor prefix corresponds to the stock symbol (ONNN) for ON
Semiconductor Corp.
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: NRob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -34,6 +34,7 @@ national	National Semiconductor
 nintendo	Nintendo
 nvidia	NVIDIA
 nxp	NXP Semiconductors
+onnn	ON Semiconductor Corp.
 picochip	Picochip Ltd
 powervr	Imagination Technologies
 qcom	Qualcomm, Inc.