dt-bindings: Add a prefix for Gateworks Corporation

As I am adding a PLD GPIO driver for Gateworks, we need
their prefix among the vendors.

Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -141,6 +141,7 @@ focaltech	FocalTech Systems Co.,Ltd
 friendlyarm	Guangzhou FriendlyARM Computer Tech Co., Ltd
 fsl	Freescale Semiconductor
 fujitsu	Fujitsu Ltd.
+gateworks	Gateworks Corporation
 gcw Game Consoles Worldwide
 ge	General Electric Company
 geekbuying	GeekBuying