dts: arm: stm32 devices have lptim irq priority set to 1

The priority of the LPTim interrupt on each stm32 soc
is put at the same level as the SysTick. This priority
level is given by the system Handler priority reg
(SHPR3) when the PM is not configured.

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