dt-bindings: Add vendor prefix for VXT Ltd

VXT Ltd is a manufacturer of projected capacitive touch panel
and display solutions: http://www.vxt.com.tw/Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190219002706.20077-1-festevam@gmail.com

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -442,6 +442,7 @@ vivante	Vivante Corporation
 vocore VoCore Studio
 voipac	Voipac Technologies s.r.o.
 vot	Vision Optical Technology Co., Ltd.
+vxt	VXT Ltd
 wd	Western Digital Corp.
 wetek	WeTek Electronics, limited.
 wexler	Wexler