rtc: add phytium rtc driver document

phytium inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7V7W5


CVE: NA

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RTC driver function support for Phytium desktop and embedded CPUs

Reviewed by: Wang Nan <wangnan1505@phytium.com.cn>
Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>