There's no so much differences between different families of STM32.
Manual (migrate STM32F1 -> STM32F2) says, that GPIO and Flash are
significantly different, the other interfaces (we use) are almost
the same. So, let's name this arch as 'stm32', not 'stm32f2' (as we
do this in linux).

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>