sw64: add set time support for hypervisor based rtc

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56WV8



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Calculate the offset between target time and current time, then
add it to current time in read_time method. with this support,
writable requirements of rtc clock of guest os can be met.

Signed-off-by: Du Yilong <duyilong@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>