cortex_m: remove old target breakpoints/watchpoints

Sometimes the target may have breakpoint registers set from a previous
debug session, we can either sync them or as we have chosen here clear them.

Change-Id: I439a623ebbf010246a70e5596d04aa7d546da731
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1363
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>