[posix] get time offset between host and rcp (#5089)

To support CSL on RCP mode, POSIX host needs to know current time on
RCP. This commit adds function to periodically calculate the time
offset between host and RCP so that host can get an estimated RCP time
by adding its current time with this offset.

examples/platforms/nrf528xx/src/alarm.c

@@ -688,7 +688,6 @@ void RTC_IRQ_HANDLER(void)
     }
 }
 
-#if OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
 uint64_t otPlatTimeGet(void)
 {
     return nrf5AlarmGetCurrentTime();
@@ -698,4 +697,3 @@ uint16_t otPlatTimeGetXtalAccuracy(void)
 {
     return XTAL_ACCURACY;
 }
-#endif // OPENTHREAD_CONFIG_TIME_SYNC_ENABLE