flash/stm32l4x: STM32L5 support programming when TZEN=1 and RDP=0x55

flash/stm32l4x: STM32L5 support programming when TZEN=1 and RDP=0x55

when RDP level is 0.5 the provided work-area should reside in non-secure RAM
to ensure that:
 - add a hint in the driver level
 - reduce the usage of secure RAM only when TZEN=1 and RDP is not 0.5
   (check the target configuration file)

Change-Id: Idbf2325e609b84ef8480eefdb49a176fdf7e07c7
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6035
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Tested-by: jenkins