EMMC: add dts bindings documents

ascend inclusion
category: feature
feature: Ascend emmc adaption
bugzilla: https://gitee.com/openeuler/kernel/issues/I4F4LL


CVE: NA

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This file documents differences between the core Synopsys dw mshc hi3xxx
controller properties described by hi3xxx-dw-mshc.txt and the properties
used by the Hisilicon hi3xxx specific extensions to the Synopsys Designware
Storage Host Controller.

Signed-off-by: zhangguijiang <zhangguijiang@huawei.com>
Reviewed-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>