We currently have a generic port for Nvidia Tegra 2/3/4 devices using U-Boot as bootloader, supporting 9 different devices at the time of writing. This wouldn't have been possible without the work of Svyatoslav Ryhel (also known as Clamor), who has been working on Tegra devices for the last few years and is also a maintainer of Nvidia Tegra SoCs in U-Boot.
The wiki page has a list of supported devices.
Svyatoslav ported most of these devices without owning one, just relying on testing from people. This means anyone having such a device can reach him and eventually will be able to replace the old proprietary vendor bootloader with U-Boot.
If you're interested, you can reach out Svyatoslav in #postmarketOS-on-tegra:matrix.org matrix room or e-mail him to get personal contacts at clamor95@gmail.com.