soc: stm32l1: Add support for STM32 IWDG

The driver for STM32's independent watchdog already exists and is
compatible with the stm32l1 SoC. Enable the independent watchdog
for the stm32l1 series for use with this driver.

Signed-off-by: Kwon Tae-young <tykwon@m2i.co.kr>