This boolean function simply returns whether or not the runtime
status of the device is 'active'. The typical scenario is driver
calls pm_runtime_get firstly, then check pm_runtime_active in
atomic environment.

Also add entry to Documentation/power/runtime.txt
Signed-off-by: NYanmin Zhang <yanmin.zhang@intel.com>
Signed-off-by: NShuoX Liu <shuox.liu@intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>