devicetree: leds: Introduce "panic-indicator" optional property

It's desirable to specify which LEDs are to be blinked on a kernel
panic. Therefore, introduce a devicetree boolean property to mark
which LEDs should be treated this way, if possible.

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

Documentation/devicetree/bindings/leds/common.txt

@@ -37,6 +37,9 @@ Optional properties for child nodes:
                      property is mandatory for the LEDs in the non-flash modes
                      (e.g. torch or indicator).
 
+- panic-indicator : This property specifies that the LED should be used,
+		    if at all possible, as a panic indicator.
+
 Required properties for flash LED child nodes:
 - flash-max-microamp : Maximum flash LED supply current in microamperes.
 - flash-max-timeout-us : Maximum timeout in microseconds after which the flash