I have gone through the spec and there is only one line written about it that is "Write response must always follow the last write transfer" and I have also gone through many other sites. all the time I have come on to the above conclusion but still it is not clearly written
"write response be always on next clock edge of Wlast signal or there can be delay between wlast and Write response."
It is definitely possible have delay between WLAST and write response.Please note that if write response has to be given always in next cycle of WLAST then there is no need of BVALID.
Only mentioned limitation in protocol is that in any case write response can not occur before WLAST (last data beat).