flash/stm32l4x: introduce auto-probe when OPTR is changed

auto re-probing is ensured by having optr cache set in the
last probe operation.

this will help to detect if flash options have been modified by the
running application or by the user using direct register access.

Change-Id: I05cd7ab9e83a7fc26ac6cff175b3c11b0efa2eb5
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/5935
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Tested-by: jenkins