dts: bindings: stm32 uart gives the EXTI line nb for PM

When the EXTI Line nb is not set as interrupt by default
(at reset value), the uart instance cannot waekup the
system from its low power stop mode.
The EXTI line must be specified, like with stm32WL55

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