drivers: counter: stm32: Fix alarm time calculation

The calculated alarm time starts from 2000 but the gmtime_r needs as
input the time from epoch (1970). This causes the alarm time to be
miscalculated due to leap years, as 2000 is a leap year and 1970 is not.
To fix the issue, the 2000 timestamp can be added to the input time of
gmtime_r.

Fixes #32260

Signed-off-by: Antonis Sioutas <antonis.si510@gmail.com>