flash/stm32l4x: add support of STM32WB1x

flash/stm32l4x: add support of STM32WB1x

STM32WB1x devices has a single flash bank up to 320 KB (page 2KB)

note: STM32WB5x/WB3x are single banks as well but do have 4KB as page size.
note: remove the assert that checks if max_mages is power of two, because
      STM32WB1x flash size is not a power of 2

Change-Id: Ib514cf989ecb819d25d1c4a65d641d0a1a3d9f18
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6129
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>