[efr32] Radio: implement Rx timestamping (#4482)
[efr32] Radio: implement Rx timestamping (#4482)

bukepo/openthread,jwhui/openthread,openthread/openthread,librasungirl/openthread,abtink/openthread,srickardti/openthread,lanyuwen/openthread,abtink/openthread,bukepo/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,openthread/openthread,srickardti/openthread,librasungirl/openthread,chshu/openthread,librasungirl/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,lanyuwen/openthread,librasungirl/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,jwhui/openthread,chshu/openthread,bukepo/openthread,chshu/openthread,lanyuwen/openthread