stm32f2x: added STM32F411xx series support.

Added STM32F411xx series to flash driver.

Tested on NUCLEO-F411RE board(STM32F411RET6).

Change-Id: Id7d1f2858c09815a013e0590e65ad193fb039157
Signed-off-by: Nemui Trinomius <nemuisan_kawausogasuki@live.jp>
Reviewed-on: http://openocd.zylin.com/2258
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>