drivers: timer: stm32_lptim: Fix behavior of st,timeout

To be consistent with the definition of label stm32_lp_tick_source,
be sure "st,timeout" is taken from the LPTIM defined as lp_tick_source.

Then, fix the computation of lptim_time_base.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>