Fix debug_monitor code for older ARMs (ARM11)

• Enable monitor mode prior to accessing watchpoint registers for v6, v6.1 architectures.
• Fix configuration scheme for v6, v6.1 and v7 Debug Archs
• Enable monitor unconditionally and for good instead of enabling and disabling it (needed for single stepping on on v6/v6.1)

Tested on RPI-B and Arndale

Differential Revision: https://reviews.freebsd.org/D6008