gpio-poweroff using adrv9009 zu11eg RF-SOM and ADRV2CRR-FMC carrier board

Hello, I've a system including an adrv9009 zu11eg RF-SOM plugged on a ADRV2CRR-FMC board running Linux . The whole subsystem is connected to a host via a cabled PCIe connection.

Both PCIe adapter and adrv9009 zu11eg RF-SOM - ADRV2CRR-FMC are powered using a ATX24  power supply. The power supply switches on automatically as soon as the host system starts (using PS_ON wire) ... 

I need to implement a mechanism which switches off the power supply (provided to PCIe adapter, ADRV2CRR-FM and adrv9009 zu11eg) Linux has completed the shutdown process.

As far as I understood this is topically achieved using the "gpio-poweroff" driver ... with can be used to disconnect the PS_ON of the power supply from GND ...causing the power off of the power supply.

I tried to add the "gpio-poweroff" stuff to the device tree but at boot time the system is informing me that gpio-poweroff can't be assigned since pm_power_off  is already used elsewhere... I guess ADI used gpio-poweroff to switch off the power supply at adrv9009 zu11eg or ADRV2CRR-FM level.

"root@petalinux-test:~# dmesg | grep gpio-poweroff
[    2.444566] poweroff-gpio fpga-axi@0:gpio-poweroff: gpio_poweroff_probe: pm_power_off function already registered
[    2.444582] poweroff-gpio: probe of fpga-axi@0:gpio-poweroff failed with error -16
root@petalinux-test:~#
"

Is this correct ? Do you turn off the power at ADRV2CRR-FM or at adrv9009 zu11eg level ?

Which GPIO has been used for implementing the gpio-poweroff mechanism?

Is this signal physically  accessible/measurable anywhere on ADRV2CRR-FM or adrv9009 zu11eg ?

Thanks for any advice...

Joel