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>