cortex_m: allow setting debug ap during create

This patch adds a Cortex-M private configuration option
that allows setting the acess point during target
creation. This circumvents situations in hybrid systems
when the correct access point can not be automatically
detected.

Change-Id: If313a5250e6e66509bb9080f3498feab7781dced
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3639
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

doc/openocd.texi

@@ -4152,6 +4152,10 @@ The value should normally correspond to a static mapping for the
 scan and after a reset. A manual call to arp_examine is required to
 access the target for debugging.
 
+@item @code{-ap-num} @var{ap_number} -- set DAP access port for target,
+@var{ap_number} is the numeric index of the DAP AP the target is connected to.
+Use this option with systems where multiple, independent cores are connected
+to separate access ports of the same DAP.
 @end itemize
 @end deffn