drivers: timer: stm32 lptim stops counting on timeout FOREVER

When setting a timeout K_TICKS_FOREVER,the lptimer clock is stopped
(no reset of the lptim).
Then is the lptim possibly re-started when another source asks for.
The lptim clock must then be re-started and continue counting.
This is the case when wakeup from sleep mode, for example.

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