target/smp: use a struct list_head to hold the smp targets

target/smp: use a struct list_head to hold the smp targets

Instead of reinventing a simply linked list, reuse the list helper
for the list of targets in a smp cluster.
Using the existing helper, that implements a double linked list,
makes trivial going through the list in reverse order.

Change-Id: Ib36ad2955f15cd2a601b0b9e36ca6d948b12d00f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6783
Tested-by: jenkins