leds: triggers: add device attribute support

As many triggers use device attributes, add support for these in
led_trigger_set which allows simplifying the drivers accordingly.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>