arm: implement kdb watchpoint functions

Implement wrappers around the existing debug_monitor interface, to be
consumed by MI kernel debugger code.

For now, the various db_printf() calls in this code remain. In the
future, they could be converted to printf() or removed altogether, to
properly decouple the DDB and GDB options.

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