[ARM] Release notes for the Custom Datapath Extension (CDE)

Summary:
This change mentions CDE assembly in the LLVM release notes and CDE
intrinsics in both Clang and LLVM release notes.

Reviewers: kristof.beyls, simon_tatham

Reviewed By: kristof.beyls

Subscribers: danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78481