This patch reduces power consumption by allowing idle devices to go to a
low power state after another device on the same power resource has been
awakened.

A power resource may be shared by multiple devices.  When all devices
sharing a power resource are put into D3_COLD state, the power resource
will be turned off.  When one of the devices is awakened, the power
resource will be turned on and all devices sharing it will be powered on to
D0uninitialized state.  These devices should be resumed, so that they have
the opportunity to go to low power state later.

[bhelgaas: changelog]
Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>