doc/device-tree-bindings/leds/common.txt

Common leds properties.

Optional properties for child nodes:
- label : The label for this LED.  If omitted, the label is
  taken from the node name (excluding the unit address).

- linux,default-trigger :  This parameter, if present, is a
    string defining the trigger assigned to the LED.  Current triggers are:
     "backlight" - LED will act as a back-light, controlled by the framebuffer
		   system
     "default-on" - LED will turn on (but for leds-gpio see "default-state"
		    property in Documentation/devicetree/bindings/gpio/led.txt)
     "heartbeat" - LED "double" flashes at a load average based rate
     "ide-disk" - LED indicates disk activity
     "timer" - LED flashes at a fixed, configurable rate

Examples:

system-status {
	label = "Status";
	linux,default-trigger = "heartbeat";
	...
};