arm64: Make register definitions const

No functional change intended.

Reviewed by: andrew
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc. (hardware)
Differential Revision: https://reviews.freebsd.org/D40502

(cherry picked from commit 00bcb77ebcbcb89bd27205b289699d711a0caf3d)