cortex_a: do not try to use MMU for translation if it wasn't enabled on target stop

On a target where AHB AP memory access is unavailable, care should be
taken to avoid treating addresses as virtual if the MMU was disabled
at the time the target was stopped.

Without this it's impossible to peek memory with Gdb when debugging
e.g. a bootloader because cortex_a8_read_memory() unconditionally
tried (and failed because of a sanity check in cortex_a8_mmu_modify)
to enable MMU.

Change-Id: Id7c63f4912920fb71a6104226ec6428d18c96a56
Reported-by:  <mbm@openwrt.org>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1787


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>