dm: led: Support "default-state" property

Add support for the device tree property "default-state". This feature
might be useful for LEDs indicating "power on" or similar states.

Note: Even with this commit gpio-leds remain in reset state. That's
because the led_gpio is not probed until DM_FLAG_ACTIVATED is set.

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>