flash/stm32l4x: fix dual bank support for STM32L552xC devices

flash/stm32l4x: fix dual bank support for STM32L552xC devices

For STM32L552xC devices with 256K flash:
  dual bank mode is activated if DB256 is set
  page size is 2KB if DBANK is set

For parts with 512K (aka STM32L5x2xE):
  DBANK controls both of dual/single bank mode and page size as well.

Change-Id: I8be668d5552fefe81acffaf2e3e35ef5e938162e
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reported-by: Patrik Bachan <diggit@users.sourceforge.net>
Fixes: https://sourceforge.net/p/openocd/tickets/317/
Reviewed-on: https://review.openocd.org/c/openocd/+/6538
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>