dt-bindings: mpam: refactor device tree node structure

arm64/mpam: refactor device tree structure to support multiple
devices

ascend inclusion
category: feature
bugzilla:
https://gitee.com/openeuler/kernel/issues/I49RB2


CVE: NA

---------------------------------------------------

To support multiple mpam device nodes, all nodes should be organized
as child of the same parent nodes. This makes sure that the mpam
discovery start and complete procedure in the right execution order.
Add modification in the devicetree documentation to record this.

Signed-off-by: Xingang Wang <wangxingang5@huawei.com>
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Cheng Jian <cj.chengjian@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>