dwmac:add phytium dwmac driver DT binding docs

phytium inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8ARED


CVE: NA

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

Add Phytium dwmac driver DT binding docs.

Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Signed-off-by: wangzhimin <wangzhimin1179@phytium.com.cn>