The tegra 12x series is the Tegra K1 (the Jetson TK1 is specifically a tegra124). So this won’t work on the TX1 (TX1 is a tegra210).
I don’t know about the specific change, but I’d suggest exploring the reverse compile of the existing DTB file, possibly editing that, and then recompiling it. Your “/boot/extlinux/extlinux.conf” should name the file with the FDT key/value entry, such as this, which also names where the file is found:
Reverse compile, edit, and recompile is similar to this:
dtc -I dtb -O dts -o /tmp/extracted.dts /boot/the_firmware_in_extlinux.dtb
# Explore, edit, so on...then recompile:
dtc -I dts -O dtb -o /tmp/modified.dtb /tmp/extracted.dts
I’d recommend not overwriting the original file and instead using an alternate file name. Then add a second boot entry in extlinux.conf which only differs by the FDT entry. If you use serial console you can then choose to boot to this entry and leave the original in place in case it doesn’t work. It can be a pain if the edit causes a boot failure and you don’t have serial console to fix it (even with serial console it is a pain if the original boot entry is gone).