flash/stm32h7x: fix bank sizes for devices with trimmed flash

flash/stm32h7x: fix bank sizes for devices with trimmed flash

STM32H7yxxI: dual independent 1 MByte banks
STM32H7yxxG: dual independent 512 Kbyte banks
STM32H7yxxB: single 128 Kbyte bank

where y = [4/5] or [A/B]

references: (documents are available in www.st.com)
 - STM32H7[4/5]x[G/I] : DS12110 Rev 7
    >> 3.3.1 Embedded Flash memory
 - STM32H750xB : RM0433 Rev 6
    >> Table 11. Flash memory organization on STM32H750xB devices
 - STM32H7[A/B]x[B/G/I] : RM0455 Rev 3
    >> 4.3.4 Flash memory architecture and usage

Change-Id: Ic9346964ef2554abf47f5832e25adfdc77bd323e
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5442
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>