LED matrix overlay

[wintersteiger]

The BBC micro:bit has a 5x5 LED matrix which is defined as a display in the board overlay files:

led_matrix: led_matrix {
  compatible = "nordic,nrf-led-matrix";
  ...
  row-gpios = <&gpio0 21 GPIO_ACTIVE_HIGH>,
	  <&gpio0 22 GPIO_ACTIVE_HIGH>, ...;
  col-gpios = <&gpio0 28 GPIO_ACTIVE_LOW>,
	  <&gpio0 11 GPIO_ACTIVE_LOW>, ...;
};

I would like to use only one of the LEDs. How do I write an overlay that gives me a single pin alias for only one of them? It looks like I can use the syntax <&led_matrix 00>, but I can't figure out where that should go.