soc: arm: stm32 adjust wdt timer due to LSI oscillator characteristics

Due to a wide range of variuation in the LSI RC oscillator
characteristics given by the datasheet of the soc,
It is necessary to add a delay for hardware watchdog.
This is done by the CONFIG_TASK_WDT_HW_FALLBACK_DELAY

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