Custom Device Tree Overlay Problem (Allwinner sun50i-h6, Orange Pi 3 LTS, Armbian)

Question

I'm trying find the current syntax for a custom DT Overlay for Armbian OS on an Orange Pi 3 LTS.

This overlay is meant to set a specific GPIO pin (PD15) as Output HIGH on bootup.

I create the following code based on various references scattered online:

/dts-v1/;
/plugin/;

/ {
    compatible = "allwinner,sun50i-h6";

    fragment@0 {
        target = <&leds>;
        __overlay__ {
            mps_led: led {
                  label = "mps_led";
                  linux,default-trigger = "default-on"; 
                  pinctrl-names = "default";
                  gpios = <&pio 3 15 0>; /* PD15 */
                  function = "gpio_out";
                  default-state = "on";
                  status = "okay"
                };
        };
    };
};

I used the 'leds' device class as it was recommended to use an existing class for the DT hook rather than try to create a new one.

The code compiles from *.dts to *.dto and gets added to armbianEnv under user-overlay as expected, but doesn't work (on bootup the pin remains OFF and LOW).

Can anyone please help with correcting the syntax and/or other mistakes in the above to make it work?