buf_ring: Add an Arm copyright

I've change enough of this file to add Arm as a copyright holder.
Add it after the "All rights reserved" line as that's not needed.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46157

(cherry picked from commit 87940d2b331c2575f96fbff2600d8da35191157b)