drivers: rtc: sam: Fix missing function

The #64939 introduced a few convenience function like
rtc_utils_validate_rtc_time. However the PR did not replace all
occurrences which result on a build error. This add the missing
header include to remove a building warning and replace the old
function by the new one.

Fixes #81454

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>