cortex_a: select APB-AP as the default AP

cortex_a: select APB-AP as the default AP

Debug initialization blindly selects AP#0 as default, which is the AHB-AP
in many cases. This sets the default for target_read/write functions.
However, AHB-AP is the wrong choice, because it bypasses caches on read
and write and also makes some peripherals inaccessible (e.g. l2 outer
caches). This patch explicitely selects the APB-AP (debug_ap) as the
default.

Change-Id: I13f9e0750186d35dcfc135c8d67d437c5884d9c4
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3113
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>