dt-bindings: Add vendor prefix for csky

Add csky vendor definition.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -84,6 +84,7 @@ cosmic	Cosmic Circuits
 crane	Crane Connectivity Solutions
 creative	Creative Technology Ltd
 crystalfontz	Crystalfontz America, Inc.
+csky	Hangzhou C-SKY Microsystems Co., Ltd
 cubietech	Cubietech, Ltd.
 cypress	Cypress Semiconductor Corporation
 cznic	CZ.NIC, z.s.p.o.