dt-bindings: add openrisc to vendor prefixes list

Add OpenRISC.io to vendor prefixes.  This is reserved for softcores
developed by the OpenRISC community.  The OpenRISC community has
separated from OpenCores.org requiring a new prefix.
Reviewed-by: Andreas Färber <afaerber@suse.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -246,6 +246,7 @@ onion	Onion Corporation
 onnn	ON Semiconductor Corp.
 ontat	On Tat Industrial Company
 opencores	OpenCores.org
+openrisc	OpenRISC.io
 option	Option NV
 ORCL	Oracle Corporation
 ortustech	Ortus Technology Co., Ltd.