armv8_dpm: retrieve only necessary registers on halt

armv8_dpm: retrieve only necessary registers on halt

to speed up debugging, don't load the complete register context
on a halt event, load only those registers that might be
clobbered during debugging.

Change-Id: I0b58e97aad6f28aefce4a52e870af61e1ef1a44f
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3995
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>