leds: lp5521: Add multicolor framework multicolor brightness support

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

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>