drivers: timer: lptim timer clock on stm32u5 has a prescaler

The stm32 devices shows a x2 factor on the LPTIM1,3,4
clock source but it acts as a prescaler.
The max lptim counter (timebase) is counting 4 sec
In that case, the LPTIM count unit is double.

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