leds: gpio: use OF variant of LED registering function

In leds-gpio we support LEDs specified in DT so we should use
(devm_)of_led_classdev_register. This allows passing DT node as argument
for use by the LED subsystem.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>