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