arm_adi_v5: Convert the AP references from numbers to pointers

arm_adi_v5: Convert the AP references from numbers to pointers

Change the debug_ap and memory_ap fields of the cortex_a target and
the debug_ap field of the cortex_m target to be pointers to the
struct adiv5_ap instead of AP numbers in some known DAP.

This reduces the dependency on the DAP struct in the targets and
enables MEM-AP accesses to take the relevant AP as parameter.

Change-Id: I39d7b134d78000564b7eec5bff464adf0ef89147
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3147
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>