> Weird, since the HiKey 960 only has a compact PCIe M.2 slot.

Better that than no PCIe at all, I guess.

> thus far, there don't seem to be any ARM boards with full sized PCIe expansion slots.

There're ARMv8 server boards with one or several PCIe x16 slots (from Gigabyte, HP, Avantek, E4 and others, mainly using X-Gene or ThunderX). In the sub-US$800 range there're (beside the LeMaker Cello with botched PCIe x16) at least these boards with PCIe x4:

- Nvidia Jetson TX1 (Cortex-A57)
- Nvidia Jetson TX2 (Denver2 + Cortex-A57)
- Marvell MacchiatoBIN (Cortex-A72)