docs: update stm32f1x/stm32f2x driver info

As we use the two ST flash drivers for multiple stm32 variants update the
docs as to which targets use which driver.

Change-Id: I84943ff45482a22b3d3dd8491bb4242d79415939
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/990
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>