I found now there have bug, though it can toggle GPIO pins
hikey960:/sys/class/gpio # ls
export gpiochip320 gpiochip368 gpiochip416 gpiochip464 unexport
gpiochip280 gpiochip328 gpiochip376 gpiochip424 gpiochip472
gpiochip288 gpiochip336 gpiochip384 gpiochip432 gpiochip480
gpiochip296 gpiochip344 gpiochip392 gpiochip440 gpiochip488
gpiochip304 gpiochip352 gpiochip400 gpiochip448 gpiochip496
gpiochip312 gpiochip360 gpiochip408 gpiochip456 gpiochip504
So you can that the GPIO is not sequential, I found actually it's reversed the order, so gpiochip280 is corresponding to GPIO28 controller, gpiochip504 is corresponding to GPIO0 controller, etc. There must have some thing wrong, I will report this to bugs tracking system.
Anyway, you can do a quick try as below, I use GPIO_021 as example, it's easily to probe with multimeter:
GPIO_021 is registers into gpiochip488 group, so GPIO_021 has logical number is 493:
echo 493 > export
echo out > gpio493/direction
echo 1 > gpio493/value
So I can see the output to high level.