dt-bindings: leds: gpio: Add optional retain-state-shutdown property

On Baseboard Management Controller (BMC) systems it's sometimes
necessary for a LED to retain its state across a BMC reset (which is
independent of the host system state). Add a devicetree property to
describe this behaviour. The property would typically be used in
conjunction with 'default-state = "keep"'.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Brandon Wyman <bjwyman@gmail.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>

Documentation/devicetree/bindings/leds/leds-gpio.txt

@@ -18,6 +18,9 @@ LED sub-node properties:
   see Documentation/devicetree/bindings/leds/common.txt
 - retain-state-suspended: (optional) The suspend state can be retained.Such
   as charge-led gpio.
+- retain-state-shutdown: (optional) Retain the state of the LED on shutdown.
+  Useful in BMC systems, for example when the BMC is rebooted while the host
+  remains up.
 - panic-indicator : (optional)
   see Documentation/devicetree/bindings/leds/common.txt