cortex_a: force cache and tlb bypass when cpu is in debug state

cortex_a: force cache and tlb bypass when cpu is in debug state

for minimal impact on the hardware state, force all memory accesses to
bypass the caches and tlbs. This may actually be the default, but ARM
recommends in DDI0406C to set proper default values on debug init.

Change-Id: If5ac097b6ee725c047b1e86c2f90eabe16b98c7b
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3079
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins