arm64: Add the pointer auth registers to armreg.h

Add the pointer authentication registers to armreg.h. These will be
used to support pointer authentication in a kernel built with GCC.

Reviewed by: jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45262

(cherry picked from commit 57d714a23f5ce21e389d53636b8bc6c1b45d518e)