stlink: support srst reset

stlink: support srst reset

This adds the ability to support srst reset for the stlink/v2.
stlink/v1 will fallback to using SYSRESETREQ which is a full reset - including peripherals.

To enable the use of the srst add the following to your cfg:
reset_config srst_only

Change-Id: I570de607c5f370fd6a4abf47360686c9be07bcdd
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/581
Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>