drivers: timer: lptim is [EXPERIMENTAL] for stm32 soc series only

Activation of the LPTIMER is valid for SLEEP MODE only
The choice of the lptim clock source is STM32_LPTIM_CLOCK
set the LSE in first position to have as default value

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