If this is running under L4T then the Linux kernel already has some things set up for this. It is possible that the register you are dealing with is also being set up by the kernel. If watchdog software is already set up, you will have this file:
/dev/watchdog
The existing Linux implementation monitors whether something reads the watchdog files. If the file is opened for read, the timer runs. If something writes to the file, the timer is reset. Doing this for a short time will result in the JTX1 rebooting:
sudo tail -f /dev/watchdog
To prevent reboot you could do something like this:
sudo echo 1 > /dev/watchdog
Likewise, killing the “tail -f” would also cancel reboot.
The kernel driver for this would probably be a good template for watchdog code under Linux.