NVIDIA does have a 64-bit ARM chip in products today; the Tegra X1. Unfortunately they don't seem to have a dev kit available for it. It's a really interesting design too, a completely custom core that can use hardware or software translation of ARM instructions to its own internal instruction set, like Transmeta. https://en.wikipedia.org/wiki/Project_Denver

It's speculated that NVIDIA wanted it to support both ARM and x86, but was unable to license x86 from Intel.