stm32f2x flash: add STM32F401 (F4 low power)

stm32f2x flash: add STM32F401 (F4 low power)

Add support for the new STM32F401 parts.  These are similar to the
STM32F405/407 however they are a new Low Power variant with ID code
0x423 and have 256K of Flash.  Tested with a modified F4 discovery
board.

Change-Id: Ida5fb14a0832934b4d6d1ec11e602df5076edbc8
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/1521
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>