tests: drivers: watchdog wwdg testing on stm32 boards

Define a common overlay for all the stm32 boards where the
tests/drivers/watchdog/wdt_basic_api is running.
The RCC  apb1 clock freq is reduced to pass the test
except on stm32H7 family : special overlay file.

Signed-off-by: Francois Ramu <francois.ramu@st.com>