dt-bindings: Add vendor prefix for Xinpeng Technology

Shenzhen Xinpeng Technology Co., Ltd produces for example display panels.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224112907.30758-1-heiko@sntech.de