devicetree: bindings: Add vendor prefix for Oki

Already in use for "oki,ml86v7667".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -197,6 +197,7 @@ nuvoton	Nuvoton Technology Corporation
 nvidia	NVIDIA
 nxp	NXP Semiconductors
 okaya	Okaya Electric America, Inc.
+oki	Oki Electric Industry Co., Ltd.
 olimex	OLIMEX Ltd.
 onion	Onion Corporation
 onnn	ON Semiconductor Corp.