armv8_dpm: fix exception handling

after handling of an exception in debug state, immediately
restore the original core state.

Change-Id: Ie53b63c9f19815f717f4df4390fbc13f0a204cc2
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3996


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>