devicetree: add Onion Corporation vendor id

Please see https://onion.io/contact for details.
Signed-off-by: NAntony Pavlov <antonynpavlov@gmail.com>
Acked-by: NRob Herring <robh@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12883/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -172,6 +172,7 @@ nvidia	NVIDIA
 nxp	NXP Semiconductors
 okaya	Okaya Electric America, Inc.
 olimex	OLIMEX Ltd.
+onion	Onion Corporation
 onnn	ON Semiconductor Corp.
 opencores	OpenCores.org
 option	Option NV