leds: rt4505: Add support for Richtek RT4505 flash LED controller

Add support for RT4505 flash LED controller. It can support up to 1.5A
flash current with hardware timeout and low input voltage protection.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>