!4974 arm64/mpam: set default feedback of last_cmd_status interface as null string

Merge Pull Request from: @ci-robot 
 
PR sync from: Zeng Heng <zengheng4@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/4VEUXAZSOL3NGIIMGWPLOZOJP77W3VI3/ 
 
https://gitee.com/openeuler/kernel/issues/I95IFO 
 
Link:https://gitee.com/openeuler/kernel/pulls/4974

 

Reviewed-by: Liu YongQiang <liuyongqiang13@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>