PCI: host-common: Add pci_host_common_d3cold_possible() helper

Add a common helper, pci_host_common_d3cold_possible(), to determine
whether PCIe devices under host bridge can safely transition to D3cold.

This helper is intended to be used by PCI host controller drivers to decide
whether they can safely put the endpoint devices into D3cold based on their
power state and wakeup capabilities. Once the devices are transitioned into
D3cold, the host controller can be safely powered off.

The helper walks all devices on the all downstream buses and only allows
the devices to enter D3cold if all PCIe endpoints are already in
PCI_D3hot. This ensures that the host controller driver does not broadcast
PME_Turn_Off or power down the controller while any active endpoint still
requires the link to remain powered.

For devices that may wake the system, the helper additionally requires that
the device supports PME wake from D3cold (via WAKE#). Devices that do not
have wakeup enabled are not restricted by this check and do not block the
devices under host bridge from entering D3cold.

Devices without a bound driver and with PCI not enabled via sysfs are
treated as inactive and therefore do not prevent the devices under host
bridge from entering D3cold. This allows controllers to power down more
aggressively when there are no actively managed endpoints.

Some devices (e.g. M.2 without auxiliary power) lose PME detection when
main power is removed. Even if such devices advertise PME-from-D3cold
capability, entering D3cold may break wakeup. Return PME-from-D3cold
capability via 'pme_capable' parameter so PCIe controller drivers can apply
platform-specific handling to preserve wakeup functionality.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[mani: commit log and removed the device checks for d3cold]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260429-d3cold-v5-1-89e9735b9df6@oss.qualcomm.com

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