leds: max77650: Add LEDs support

This adds basic support for LEDs for the max77650 PMIC. The device has
three current sinks for driving LEDs.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>