Documentation: leds-lp5523: describe master fader attributes

Add the usage of the new attributes for master faders.

Signed-off-by: Toshi Kikuchi <toshik@chromium.org>
Acked-by: Milo Kim <milo.kim@ti.com>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>