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
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.

(cherry picked from commit 3ef68bc62c1e3ca9c452177f5cb9fd4de0df590d)