drivers: watchdog: iwdg_stm32: add support for STM32WB0

Add support for the STM32WB0 series to the existing STM32 driver for
Independent Watchdog peripheral.

NOTE: this commit only includes basic support for the watchdog.
STM32WB0 specificities such as the different possible clock sources, LSI
frequency measurement or window support are left for future implementation.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>