ARM: dts: stm32: update iwdg with lsi clock name for stm32f429

This patch updates iwdg watchdog to use lsi clock name.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>