dts: bindings: lptim stm32 has a prescaler entry for the lptim clock

This is a new parameter to divide the LPTIM input clock
by a prescaler, changing the max reachable timeout of the LP timer.
It will divide the LPTIM input clock by 1 (reset value) up to 128.
The lptim configuration register is written with a 3bit value.

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