leds: Add multicolor ID to the color ID list

Add a new color ID that is declared as MULTICOLOR as with the
multicolor framework declaring a definitive color is not accurate
as the node can contain multiple colors.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>