[nrf52840] update alarm.c for nrf52840 platform (#2602)
[nrf52840] update alarm.c for nrf52840 platform (#2602)


pvanhorn/openthread,openthread/openthread,LeZhang2016/openthread,georgecpr/openthread,openthread/openthread,jwhui/openthread,mszczodrak/openthread,georgecpr/openthread,srickardti/openthread,LeZhang2016/openthread,librasungirl/openthread,librasungirl/openthread,pvanhorn/openthread,bukepo/openthread,LeZhang2016/openthread,gandreello/openthread,jwhui/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,georgecpr/openthread,turon/openthread,LeZhang2016/openthread,abtink/openthread,bukepo/openthread,mszczodrak/openthread,pvanhorn/openthread,bukepo/openthread,erja-gp/openthread,turon/openthread,pvanhorn/openthread,pvanhorn/openthread,lanyuwen/openthread,erja-gp/openthread,gandreello/openthread,LeZhang2016/openthread,gandreello/openthread,srickardti/openthread,turon/openthread,openthread/openthread,lanyuwen/openthread,mszczodrak/openthread,erja-gp/openthread,pvanhorn/openthread,chshu/openthread,mszczodrak/openthread,jwhui/openthread,chshu/openthread,openthread/openthread,srickardti/openthread,erja-gp/openthread,georgecpr/openthread,georgecpr/openthread,LeZhang2016/openthread,jwhui/openthread,srickardti/openthread,abtink/openthread,mszczodrak/openthread,turon/openthread,librasungirl/openthread,lanyuwen/openthread,georgecpr/openthread,chshu/openthread,erja-gp/openthread,chshu/openthread,gandreello/openthread,gandreello/openthread,lanyuwen/openthread,mszczodrak/openthread,erja-gp/openthread,chshu/openthread,abtink/openthread,gandreello/openthread,bukepo/openthread,abtink/openthread,lanyuwen/openthread