leds: leds-core: Implement the retain-state-shutdown property

Read the retain-state-shutdown device tree property to set the
existing LED_RETAIN_AT_SHUTDOWN flag. Then check the flag when
unregistering, and if set, don't set the brightness to OFF. This
is useful for systems that want to keep the HW state of the LED
across reboots.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>