stm32w: Added sample target configuration for STM32W108 with STLink-V2

As requested, here is the target configuration that I'm using for an
STLink-V2-attached STM32W108C8. For some reason, it only seems to work
with "reset_config trst_only".

Change-Id: Icbff4f83343e1f505d8afdfc53ff6f8b7496cac9
Signed-off-by: Ben Nahill <bnahill@gmail.com>
Reviewed-on: http://openocd.zylin.com/1347
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>