boards: arm: stm32 boards with lptimer set node stm32_lp_tick_source

Change the name of the node for the lptim used as lowpower
tick source to stm32_lp_tick_source.
Once enabled, this node is known as stm32_lp_tick_source
That will avoid naming the node lptim1 or lptim2 or lptim, etc.

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