flash/stm32l4x: introduce stm32l4_part_info.flags for devices features

flash/stm32l4x: introduce stm32l4_part_info.flags for devices features

instead of adding a new member into stm32l4_part_info for every relevant
feature, .flags serves as container for the devices' features.

identified features: F_HAS_DUAL_BANK, F_USE_ALL_WRPXX, F_HAS_TZ

Change-Id: I3093e54c6509dec33043ebe6f87675198bf1967a
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5540
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>