dt-bindings: leds: Add retain-state-shutdown boolean

Document the retain-state-shutdown property that indicates that a LED
should not be turned off or changed during system shutdown.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>