flash/stm32l4x: probe tzen and rdp values

flash/stm32l4x: probe tzen and rdp values

introduction of 'enum stm32l4_rdp' enumerating possible RDP levels for
devices with and without TrustZone.

also in 'stm32l4_flash_bank' structure we added and rdp and tzen members
to store read values by the helper 'stm32l4_sync_rdp_tzen'

these new members are used to display security and protection status
while probing the flash.

Change-Id: Icf883189715278a3323fe210d295047678b16592
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5541
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>