x_mem_result_ready needs to be added for subsystem sharing

In order to support the sharing of a subsystem between multiple cores using the x-interface, an x_mem_result_ready would have to be added. The current spec states that there is no ready because the subsystem must be ready to accept the memory result at any time. With multiple cores, this cannot be guaranteed as it would then even be possible for multiple cores to raise the x_mem_valid at the same time. The subsystem would in this case only be able to react to one of those data packets, violating the property that it must be ready to accept the memory result at any time for every core raising the valid.