dt-bindings: mpam: add document for arm64 mpam

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


CVE: NA

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Add devicetree bindings document for arm64/mpam, this add basic
description for the main properties of mpam devicetree definition.

Signed-off-by: Xingang Wang <wangxingang5@huawei.com>
Reviewed-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>