leds: move default_state read from fwnode to core

This patch introduces a new function to read initial
default_state from fwnode.

Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Denis Osterland-Heim <Denis.Osterland@diehl.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>