PCI: qcom: Add D3cold support

Add support for transitioning PCIe endpoints under host bridge into D3cold
by integrating with the DWC core suspend/resume helpers.

Implement PME_Turn_Off message generation via ELBI_SYS_CTRL and hook it
into the DWC host operations so the controller follows the standard
PME_Turn_Off based power-down sequence before entering D3cold.

When the device is suspended into D3cold, fully tear down interconnect
bandwidth and OPP votes. If D3cold is not entered, retain existing
behavior by keeping the required interconnect and OPP votes.

Use dw_pcie::skip_pwrctrl_off to avoid powering off devices during suspend
to preserve wakeup capability of the devices and also not to power on the
devices in the init path.

Finally, drop the qcom_pcie::suspended flag and rely on the existing
dw_pcie::suspended state, which now drives both the power-management flow
and the interconnect/OPP handling.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[mani: commit log]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260429-d3cold-v5-5-89e9735b9df6@oss.qualcomm.com

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