dt-bindings: documenting compatible string vendor "visionox"

Documenting compatible string vendor "visionox" in vendor-prefix yaml file.

Signed-off-by: Harigovindan P <harigovi@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429054515.4976-2-harigovi@codeaurora.org