flash/stm32h7x: add support of STM32H7Ax/H7Bx devices

flash/stm32h7x: add support of STM32H7Ax/H7Bx devices

this new device has the following features:
 - single core cortex-M7
 - 2MB flash - dual bank
    - page size 8k
    - write protection grouped by 4 sectors
    - write block size 128 bits (16 bytes)

the bit definition of FLASH_CR is different than STM32H74x,
that's why we introduced a helper to compute the FLASH_CR value

Change-Id: I4da10cde8dd215b1b0f2645f0efdba9d198038d1
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5441
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>