PM / Domains: Improve handling of wakeup devices during system suspend

Kevin points out that if there's a device that can wake up the system
from sleep states, but it doesn't generate wakeup signals by itself
(they are generated on its behalf by other parts of the system) and
it currently is not enabled to wake up the system (that is,
device_may_wakeup() returns "false" for it), we may need to change
its wakeup settings during system suspend (for example, the device
might have been configured to signal remote wakeup from the system's
working state, as needed by runtime PM).  Therefore the generic PM
domains code should invoke the system suspend callbacks provided by
the device's driver, which it doesn't do if the PM domain is powered
off during the system suspend's "prepare" stage.  This is a valid
point.  Moreover, this code also should make sure that system wakeup
devices that are enabled to wake up the system from sleep states and
have to remain active for this purpose are not suspended while the
system is in a sleep state.

To avoid the above issues, make the generic PM domains' .prepare()
routine, pm_genpd_prepare(), force runtime resume of devices whose
system wakeup settings may need to be changed during system suspend
or that should remain active while the system is in a sleep state to
be able to wake it up from that state.

Reported-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

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