The system level attribute of ACPI power resources is the lowest
system sleep level (S0, S2 etc.) in which the given resource can be
"on" (ACPI 5.0, Section 7.1).  On the other hand, wakeup power
resources have to be "on" for devices depending on them to be able to
signal wakeup.  Therefore devices cannot wake up the system from
sleep states higher than the minimum of the system level attributes
of their wakeup power resources.

Use the wakeup power resources' system level values to get the
deepest system sleep state (highest system sleep level) the given
device can wake up the system from.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>