dt-bindings: display: add support for ZTE VGA device

It adds bindings doc for ZTE VOU VGA output device.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491490870-6330-4-git-send-email-shawnguo@kernel.org