CryptoCell headers

[MBradbury]

This repository seems to have had CryptoCell related headers removed from it. I was looking to use some of the CryptoCell APIs, but need to replace arch/cpu/nrf52840/lib/nrf52-sdk with the version from here in order to have these headers.

Would it be possible to add in appropriate headers (and ideally the lib files)? If not it would be useful to provide a way to download them as needed.