flash/nor/stm32l4x : add structure containers to hold devices' information

flash/nor/stm32l4x : add structure containers to hold devices' information

This rework is inspired from the 'flash/nor/stm32h7x.c'
This rework will ease the support of new devices on top of this driver:
  for example: STM32WB have different flash base and size addresses

Notes:
 - stm32l4_probe modified in order to charge the correct part_info from
   the defined stm32l4_parts according to the device id
 - stm32l4_flash_bank.bank2_start is replaced by .part_info->bank1_sectors
 - STM32_FLASH_BASE is removed , part_info->flash_regs_base will be used instead
   based on that flash register addresses are changed to offsets,
   >> stm32l4_get_flash_reg was modified accordingly
 - stm32l4_read_option and stm32l4_write_option was modified to accept an
   offset instead of an absolute address, luckily this is the commands'
   argument by default
 - stm32l4_mass_erase modifications :
     - use MER2 only on top of dual bank devices
     - wait for BUSY bit before starting the mass erase

Change-Id: Ib35bfc3cbadc76bbeaaaba9005b82077b9e1e744
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/4932
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>