leds: add DT binding for BCM6328 LED controller

This adds device tree binding documentation for the Broadcom BCM6328 LED controller.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>