Hi,
After you create the dts files from spreadsheet, please check the README file under below path to use the python script to generate the new cfg file. Also, for xavier series platform, there is no u-boot.
~/nvidia/nvidia_sdk/JetPack_4.4_DP_Linux_DP_JETSON_XAVIER_NX/Linux_for_Tegra/kernel/pinmux/t19x$ ls
addr_info.txt mandatory_pinmux.txt pinmux-dts2cfg.py README.txt
gpio_addr_info.txt pad_info.txt por_val.txt