soc: arm: stm32f4 increase IDLE stack in case of PM

When Power Management is enabled (CONFIG_PM=y),
the CONFIG_IDLE_STACK_SIZE of 320 is not enough :
Increase its size to 512.

Signed-off-by: Francois Ramu <francois.ramu@st.com>
Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>