drivers: timer: stm32 lptim can divide its input clock freq

This PR will divide the LPTIM clock freq to increase the max timeout.
Only one LPTIM instance is considered for PM timer.
The input freq becomes a fraction of the internal PCLK
source (mainly LSE clock). As the tick per sec does not change,
the minimum lptim counter must always be >0.

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