leds: leds-gpio: Set of_node for created LED devices

When working with Device Tree we iterate over children of "gpio-leds"
compatible node and create LED device for each of them. We take care of
all common DT properties (label, default trigger, state, etc.), however
some triggers may want to support more of them.

It could be useful for timer trigger to support setting delay on/off
values with Device Tree property. Or for transient trigger to support
e.g. duration property.

We obviously should handle such properties in triggers, not in generic
code. To let trigger drivers read properties from DT node we need to set
of_node to point the relevant node. This change allows using all kind of
of helpers in e.g. "activate" callbacks.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>