stlink: add JTAG speed selection

stlink: add JTAG speed selection

ST-Link uses two separate API for setting the interface speed in SWD
and JTAG.
Add the missing API for JTAG and run the proper API depending on the
selected mode.

Change-Id: I71e955e23c1d82c2ea1c8bfade7612c5a0377a74
Author: Laurent LEMELE <laurent.lemele@st.com>
Rebased-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4703
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>