DRM: Add documentation for phytium dc

phytium inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7QLIE


CVE: NA

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    This is Phytium Display Engine support,DC/DP driver patch.

Signed-off-by: Yang Xun <yangxun@phytium.com.cn>
Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Signed-off-by: lishuo <lishuo@phytium.com.cn>