leds: lp5523: Update the lp5523 code to add multicolor brightness function

Add the multicolor brightness call back to support the multicolor
framework. This call back allows setting brightness on grouped channels
in a single function.

Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>