Complementing devm_led_classdev_register add a managed version of
led_trigger_register.

I omit a managed version of led_classdev_unregister as the equivalent
devm_led_classdev_unregister isn't used in the kernel as of today.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: NJacek Anaszewski <j.anaszewski@samsung.com>