Debian is now working on the NVIDIA Jetson TX1 developer kit, a development board based on the Tegra X1 chip (Tegra 210), a 64-bit ARM chip.
We have a pre-built u-boot image in Debian as well as kernel and installer support. There are some minor kernel glitches but NVIDIA is very active upstream and I hope they'll get resolved soon.
The Jetson TX1 developer kit makes a pretty good 64-bit ARM development platform. The board is supported in mainline u-boot and the mainline kernel and NVIDIA are pretty responsive to bug reports. Unfortunately, a proprietary blob is required for USB (and Ethernet is connected via USB).
If you're interested in a good 64-bit ARM development platform, give Debian on the Jetson TX1 development kit a try.