flash/nor/stm32h7x: fix option bytes handling to work with both banks

flash/nor/stm32h7x: fix option bytes handling to work with both banks

To achieve that we need to avoid using FLASH_REG_BASE_B0, and use
bank registers instead:
   For dual bank devices, each option register is mapped in 2 addresses
   at the same offset from flash_bank_reg_base.
   This is true for OPTCR, OPTKEYR, OPTSR_CUR/PRG, OPTCCR according to
   RM0433 Rev6 (refer to section 3.9: FLASH registers)

In stm32x_write_options, according to RM0433 Rev6, after OBL launch we
should wait for OPTSR_CUR.BSY bit instead of FLASH_SR.QW

Change-Id: Ie24a91f069d03c9233797390fc2e925c737dad90
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5291
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>