PCIe HotPlug: Detect bridges that are not really HotPlug capable

Some devices report that they have an MRL when they actually
do not. Since they always report that the MRL is open, child
devices would be ignored. Try to detect these devices and
ignore their claim of HotPlug support. Specifically,
if there is an open MRL but the Data Link Layer is active,
the MRL is not real.

Revert r303645 to re-enable HotPlug support for slots with
power controllers, since it works correctly in my testing.

Start the DLL state-change timer if Presence /or/ MRL state changes,
along with other conditions. Previously, we started the timer iff
Presence changed. If there is an MRL, it must be closed for power
to be turned on, so Presence is unlikely to change on an MRL-close event.

Add a printf() of interesting registers on HotPlug interrupts and
commands (one from erj@). These were very useful for debugging.
Guard them with bootverbose, since they're spam in normal operation.

In collaboration with: jhb
Reviewed by: jhb
MFC after: 1 day
Relnotes: yes (re-enable HotPlug support for slots with power controllers)
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D7509