drivers: rtc: api: adjust callback APIs to not be syscalls

The rtc device driver APIs rtc_update_set_callback and
rtc_alarm_set_callback are not allowed from usermode threads, thus
they should not be marked as syscalls. The APIs where never
implemented as sycalls, there is no z_vrfy wrapper for them, so
only change needed is to not mark them as syscalls with the
special __syscall "attribute" and z_impl wrapper.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>