aarch64: clear CTI halt event early at debug entry

The halt event was left pending in the CTI, better to clear it immediately
after debug entry.

Change-Id: I6002f862681baf98769e3c73332a7f7f0ef938c1
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4030


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