dt-bindings: backlight: arcxcnn: Supply bindings for Arctic Sand backlight

Signed-off-by: Olimpiu Dejeu <olimpiu@arcticsand.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>