PM / hibernate: introduce system_in_hibernation

hulk inclusion
category: bugfix
bugzilla: 26326
CVE: NA

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Introduce boolean function system_in_hibernation() returning
'true' when the system carrying out hibernation.

Some device drivers or syscore need such a function to check
if it is in the phase of hibernation.

Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>