arm_adi_v5: Add a back-pointer from an AP to its DAP

arm_adi_v5: Add a back-pointer from an AP to its DAP

This will make it possible to reference directly the AP used for debug
in the target instance and remove the DAP reference. This will in turn
enable getting rid of the need to select an "active" AP in the DAP (using
dap apsel).

Change-Id: I265846a427c714204f4fd3df3cdb75843686c2d0
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3144
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>