The Snowball SBC supported by arch/arm/boot/dts/ste-snowball.dts
was made by CALAO Systems and uses the "calaosystems,snowball-a9500"
compatible. Prepare for documenting the compatible by adding
"calaosystems" to the list of vendor prefixes.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>