Below is the brightness file I found in the /sys folder. Unfortunately, I can't turn off the pwr_led light no matter what I try. I have been able to turn off the light on devices such as raspberry 3b and king3399 before, so I don't know if it is a system problem?
# root @ nanopct4 in /sys [12:13:15]
$ find . -name "*brightness*"
./devices/platform/gpio-leds/leds/status_led/brightness
./devices/platform/gpio-leds/leds/status_led/max_brightness
./devices/platform/fe330000.mmc/leds/mmc2::/brightness
./devices/platform/fe330000.mmc/leds/mmc2::/max_brightness
