samples: boards: stm32 low power blinky for stm32 boards

Set the prescaler for the stm32_lp_tick_source lptim node.
When the LPTIM is clocked by the LSE with a prescaler
of 16 (lptim freq at 2048Hz) expecting 2048 for the TICKS_PER_SEC.
When the LPTIM is clocked by the LSE with a prescaler
of 32 (lptim freq at 1024Hz) expecting 1024 for the TICKS_PER_SEC.

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