can: add phytium can driver document

phytium inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7WGOQ


CVE: NA

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add phytium can driver document

Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Signed-off-by: Chen Siyu <chensiyu1321@phytium.com.cn>