dt: bindings: Add multicolor class dt bindings documention

Add DT bindings for the LEDs multicolor class framework.
Add multicolor ID to the color ID list for device tree bindings.

CC: Rob Herring <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>