[efr32] implement otPlatTime* APIs (#3212)
[efr32] implement otPlatTime* APIs (#3212)


librasungirl/openthread,bukepo/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,turon/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,chshu/openthread,erja-gp/openthread,chshu/openthread,bukepo/openthread,chshu/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,lanyuwen/openthread,abtink/openthread,openthread/openthread,librasungirl/openthread,erja-gp/openthread,erja-gp/openthread,bukepo/openthread,bukepo/openthread,turon/openthread,chshu/openthread,turon/openthread,lanyuwen/openthread,lanyuwen/openthread,srickardti/openthread,abtink/openthread,librasungirl/openthread,erja-gp/openthread,librasungirl/openthread,lanyuwen/openthread,turon/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,erja-gp/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,erja-gp/openthread