arm64: implement kdb watchpoint functions

Add wrappers around the debug_monitor interface, to be consumed by MI
kernel debugger code. Update dbg_setup_watchpoint() and
dbg_remove_watchpoint() to return specific error codes, not just -1.

Reviewed by: jhb, kib, markj
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D29155