Give each SMP group a unique number.

Give each SMP group a unique number.

This helps e.g. if there are 8 cores, and cores 0--3 are in one SMP
group while 4--7 are in another group. (And there are 2 gdb instances
connected, one debugging the first group, and one the second.)

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: I7b6c9382eadf964529105eaf0411a42d48768668
Reviewed-on: https://review.openocd.org/c/openocd/+/6979
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>