Comment 0 for bug 1789358

===SRU Justification===
[Impact]
New TBT controllers may use Native PCIe Enumeration (i.e. just like
other normal PCIe device listed under lspci) instead of ACPI hotplug.

Since TBT controller stops getting unplugged from PCIe bus, it draws
additional power.

[Fix]
Support runtime power management for TBT controllers, so it can be put
to D3 to save powers.

[Test]
I tested and verified this patch series can make Lenovo T480's TBT
controller enters PCI D3 at runtime.

[Regression Potential]
Low. Almost all TBT controllers in the wild still uses ACPI hotplug,
this series won't affect those users.