boards: nucleo_wl55jc: Fix SYS_CLOCK_TICKS_PER_SEC when using LPTIM

When using LPTIM as tick source and since LPTIM clock source is
set as LSI (32KHz), set SYS_CLOCK_TICKS_PER_SEC to 4000 to get better
accuracy on tick computation.

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