I have not done this. In theory it is just a device tree change. Keep in mind that the serial UARTs have both a non-DMA mode available (produces devices such as “/dev/ttyS0”) and DMA modes (producing devices such as “/dev/ttyTHS2” instead of “/dev/ttyS2”). Each serial controller has a device tree section named after its base address, e.g., there are sections “serial@70006000” and “serial@70006040”.
For a given UART the modes it can be set to are in the device tree via the “compatible” line. That mode can list what drivers are allowed (and may not actually select the final driver if two are present…not sure), and for a normal ttyS# port without DMA (compatible with U-Boot) there will be “nvidia,tegra210-uart”. A mode with DMA (and not compatible with U-Boot) there will be “nvidia,tegra114-hsuart”.
To complicate this the device tree actually exists in multiple places because the kernel uses a full device tree and the boot loader uses a different device tree setup prior to the kernel ever seeing this full setup. You would need to change the device tree for both U-Boot and the kernel, not just the obvious device tree. I’m not sure where the content needs to change in order to change this with the bootloader in part because this probably changed between each release from R27.1 through R28.2 and you have not stated which release you are using.
Here are some notes on doing this which will probably need to be adjusted depending on which L4T release you are using: