soc: arm: stm32 TICKS value with LPTIM config

This is defining the SYS_CLOCK_TICKS_PER_SEC default value
depending on the LPTIM CLOCK frequency in case of LPTIMER,
to get a TICK value as a divider of the LPTIM clock source.
It gives a better result in formulas when converting
ticks to count unit.

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