drivers/wdt: Rename STM32 IWDG files to follow naming rules

Basically, all driver file names should start with the driver type as
prefix: wdt_ in case of watchdogs here.

Maybe 'iwdg' keyword could be removed entirely, and also in function
names. However that is not the scope of this patch.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>