arm64: validate breakpoint registers
In particular, we want to disallow setting breakpoints on kernel
addresses from userspace. The control register fields are validated or
ignored as appropriate.
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
(cherry picked from commit de2b9422807586d376ec7ffa7b660cd492464bdf)
(cherry picked from commit 8837e9c54072679b69ae0c0345e7ef7d241255aa)