Pinmuxing on the BBAI

[Giulio Moro]

What is the deal with pinmuxing on the AI? My understanding is you need to set the pins in u-boot (in `board/ti/am57xx/mux_data.h`), and then set them to the same value in the `/boot/dtbs/4.14.108-ti-r116/am5729-beagleboneai.dtb` file on the board.

Questions:
- is the above understanding correct?
- why?
- I see the values in `/sys/kernel/debug/pinctrl/4a003400.pinmux/pins` change according to the `/boot/dtbs/4.14.108-ti-r116/am5729-beagleboneai.dtb`, regardless of uboot. Can they be trusted?
- what is the role of ` arch/arm/dts/am5729-beagleboneai.dts` in the `u-boot` source?