leds: triggers: define module_led_trigger helper

This helps to simplify modules that provide a simple led_trigger. It's
inspired by module_platform_driver, module_i2c_driver et al.

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>