cortex_a: allow physical memory access through AHB-AP again

cortex_a: allow physical memory access through AHB-AP again

This feature is required for boards that use a programmatical way
to reset the cpu, like the TI Pandaboard with OMAP4. The board only
has a 14 pin JTAG header that doesn't feature SRST and is reset by
direct write to the PRM_RSTCTL register.

iMX6 can be reset through triggering the on-chip watchdog, but for these
methods to work reliably, access through the AHB-AP without interaction
with the CPU core is necessary.

Change-Id: I9a07a536adda83cc2f93e504384c8c7f0306220b
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3359
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>