arm_dpm: fix dpm setup

arm_dpm: fix dpm setup

When ARM64 support was being merged, a comparison ended up being
inverted. This causes NULL pointer access when target attempts to
use core cache.

Change-Id: Ic8873ddd13dbdd8100856a71b4717f44cd336e23
Signed-off-by: Girts Folkmanis <opensource@girts.me>
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4042
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>