soc: st: stm32 mcus setting the lptim clock source

Select the LPTIM clock source STM32_LPTIM_CLOCK to be
LSE or LSI depending on the DTS clocks property
of the stm32_lp_tick_source node.
This will also affect the SYS_CLOCK_TICKS_PER_SEC
depending on the lptim prescaler

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