dt-bindings: leds: Add SC27xx breathing light controller documentation

This patch adds the binding documentation for Spreadtrum SC27xx series
breathing light controller, which supports 3 outputs: red LED, green
LED and blue LED.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>