[ +chromium-os-dev ]
your question is first a CrOS porting question, although if you want to do an aarch64 userland (rather than an arm32 userland with an aarch64 kernel), you're going to be back to working on Chromium itself to get it working in the 64-bit arm world.
Chromium OS already works on arm64 kernels and we're shipping devices with it. so the basic "i want an arm64 kernel" is done. however, like any other SoC regardless of the arch, you're going to have to make sure the kernel has all the right drivers in it. "how do i port the Linux kernel" is beyond the scope of Chromium OS ... you'll need to seek help for that elsewhere (like the SoC vendor itself).
"how do i port a board" is documented here:
if you want to do an aarch64 userland, you'll run into a few OS packages failing to build. we'd like to fix them at some point, we just haven't gotten around to it as we still use an arm 32-bit userland on all systems today. patches welcome there :).
-mike