mailbox:add phytium mailbox driver document

phytium inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7VZKG


CVE: NA

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

mailbox driver support for Phytium desktop and embedded CPUs

Signed-off-by: Chen Ziqiang <chenziqiang1686@phytium.com.cn>
Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>