drivers: timer: stm32_lptim: Initialize static global variable

In some configurations, a udf instruction may be generated
when compiling code where static global variable lptim_clock_freq is
used as a divisor.
To avoid this, initialize variable on declaration so that compiler
could see that it can't be used uninitialized in this division.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>