devicetree: add Dragino vendor id

Please see http://www.dragino.com/about/about.html for details.
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12881/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -70,6 +70,7 @@ digilent	Diglent, Inc.
 dlg	Dialog Semiconductor
 dlink	D-Link Corporation
 dmo	Data Modul AG
+dragino	Dragino Technology Co., Limited
 ea	Embedded Artists AB
 ebv	EBV Elektronik
 edt	Emerging Display Technologies