devicetree: add vendor prefix for Khadas

Khadas is a new sub-brand of "Shenzhen Wesion Technology Co., Ltd.".
They are developing Amlogic and Rockchip based "DIY boxes" (single board
computers): http://khadas.com/


They are best know for their latest product: the Khadas VIM (an Amlogic
GXL S905X based SBC).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>