led: triggers: Add LED_INIT_DEFAULT_TRIGGER flag

Add the flag LED_INIT_DEFAULT_TRIGGER for indicating that trigger
being set is a default trigger for the LED class device, and
thus it should be initialized with settings provided in the fwnode.

Set the flag in the led_trigger_set_default(). It is expected to be
cleared in the activate() op of a trigger after trigger fwnode
initialization data is parsed and applied. This should happen only
once after LED class device registration, to allow leaving triggers
in the idle state on re-apply and let the users apply their own
settings without interference from the default ones.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>