flash/stm32l4x: add support of STM32WLEx devices

flash/stm32l4x: add support of STM32WLEx devices

STM32WLEx devices are based on arm Cortex-M4 running at 48MHz,
contains a single bank of maximum 256 Kbytes of flash memory.

there is 3 variants with different Flash/RAM sizes:
  STM32WLE5JC : 256K/64K
  STM32WLE5JB : 128K/48K
  STM32WLE5J8 :  64K/20K

the work-area size is set to 20 kb to fit in STM32WLE5J8

Change-Id: Ie8e186fe4be97cbc25c53ef0ade4b4dbbcee6f66
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5450
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>