dt-bindings: Add OLPC vendor prefix

One Laptop Per Child is a non-profit that produced the XO series of
eductional laptops for children.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -275,6 +275,7 @@ nxp	NXP Semiconductors
 okaya	Okaya Electric America, Inc.
 oki	Oki Electric Industry Co., Ltd.
 olimex	OLIMEX Ltd.
+olpc	One Laptop Per Child
 onion	Onion Corporation
 onnn	ON Semiconductor Corp.
 ontat	On Tat Industrial Company