stlink: add reconfigurable speed support

stlink: add reconfigurable speed support

The ability to change the speed has been added to firmware versions J22 and
above. Any attempt to change on earlier versions will be ignored without error,
as the existing code does.

For supported firmware versions the driver will attempt to get as close as
possible to supported speeds (never higher).

The default stlink speed on power up is 1.8MHz.
The driver will now also print supported clocl speeds during init.

Change-Id: Iee9bd018bb8b6f94672a12538912d41c23d48a7e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2335
Tested-by: jenkins