target: free target SMP list on shutdown

target: free target SMP list on shutdown

On SMP targets, the "target smp" command creates a list of targets
that belong to the SMP cluster. Free this list when a target gets
destroyed on shutdown. For simplicity, the complete list is free'd as
soon as the first target of the SMP cluster is destroyed instead of
individually removing targets from the list.

Change-Id: Ie217ae1efb2e819c288ff3b1155aeaf0a19b06be
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4481
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>