Are you using the devkit or custom board for Jetson Nano?
What’s your Jetpack version in use?
It seems the device tree for pinmux, which is in /hardware/nvidia/platform/t210/porg/kernel-dts/porg-platforms/tegra210-porg-pinmux-p3448-000-.dtsi.
You could refer to Updating the Bootloader Pinmux for details.