cfg: update for target's that support cortex_m AIRCR SYSRESETREQ

If the target supports SYSRESETREQ make sure we use that as the default
if srst is not fitted/configured.

Change-Id: I24c907493134506320e69c1218702930629c1cdc
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/792


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>