leds: Add LED support for MT6323 PMIC

MT6323 PMIC is a multi-function device that includes LED function.
It allows attaching up to 4 LEDs which can either be on, off or dimmed
and/or blinked with the controller.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>