stm32l4x: add OTP support for STM32 G0/G4/L4/L4+/L5/WB/WL devices

stm32l4x: add OTP support for STM32 G0/G4/L4/L4+/L5/WB/WL devices

this is a rework of #5320 started by Andreas then abandoned.

same syntax as in stm32f2x driver:

  enable OTP for writing
  > stm32l4x otp 1 enable

  write to OTP
  > flash write_bank 1 foo.bin 0
  > flash filld 0x1FFF7000 0xDeadBeafBaadF00d 1

  read OTP
  > mdw 0x1FFF7000 4

  disable OTP
  > stm32l4x otp 1 disable

Change-Id: Id7d7c163b35d7a3f406dc200d7e2fc293b0675c2
Signed-off-by: Andreas Bolsch <hyphen0break@gmail.com>
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5537
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>