aarch64: Correct target state for hardware step

aarch64: Correct target state for hardware step

When using hardware step for doing stepping, the existing DSCR records
the event as external debug request.  This will generate a SIGINT event
to GDB and causes it to stop the stepping process.
For aarch64, read DESR to check if the event is a hardware step and set
state to DBG_REASON_SINGLESTEP.
With this patch, GDB can now do source level stepping.

Change-Id: I1d06f819578c74b3ac17376c67f882adddea1f52
Signed-off-by: David Ung <david.ung.42@gmail.com>
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>