drivers: timer: stm32 lptimer adjust the TICKS_PER_SEC to LPTIM clock

With low LPTIM freq when prescaler is set to 16 or 32,
the CONFIG_SYS_CLOCK_TICKS_PER_SEC must be reduced to
LPTIM CLOCK_/prescaler to avoid spurious timer wakeup activity.
Assert error if the CONFIG_SYS_CLOCK_TICKS_PER_SEC
is not compatible with the lptim clock freq.

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