dt-bindings: leds-gpio: Add function and color properties

Refer to new "function" and "color" properties and mark "label"
as deprecated.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>