leds: gpio: Support the "panic-indicator" firmware property

Calling a GPIO LEDs is quite likely to work even if the kernel
has paniced, so they are ideal to blink in this situation.
This commit adds support for the new "panic-indicator"
firmware property, allowing to mark a given LED to blink on
a kernel panic.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>

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

@@ -23,6 +23,8 @@ LED sub-node properties:
   property is not present.
 - retain-state-suspended: (optional) The suspend state can be retained.Such
   as charge-led gpio.
+- panic-indicator : (optional)
+  see Documentation/devicetree/bindings/leds/common.txt
 
 Examples: