PCI: hotplug: Document TODOs

PCI: hotplug: Document TODOs

While refactoring the PCI hotplug core's API, I noticed a significant
amount of technical debt in some of the hotplug drivers.  Document the
issues that caught my eye for starters.

I do not have hardware at my disposal that utilizes the listed drivers
and I think that's a prerequisite to work on them to ensure that no
regressions sneak in.  But some of this hardware is so old that it may be
hard to come by.  Obviously, it is fine to support old hardware, but the
drivers need to be maintained.

If noone steps up, perhaps we should consider sunsetting a few drivers
by moving them to staging.  Based on my findings, ibmphp would be the
first candidate.  I've found it fairly difficult to apply my API
refactorings to it and have listed some obvious bugs in the driver.
cpqphp is also in need of a modernization and would be a second
candidate for relegation to staging.

shpchp was introduced in the same commit as pciehp but hasn't benefited
from the same amount of refactoring due to the decline of conventional
PCI's relevance.  Yet hardware supporting it may be more prevalent than
for the proprietary hotplug methods.

Per Documentation/process/2.Process.rst, "a TODO file should be present"
for drivers in staging.  The file introduced by the present commit may
serve as a basis for this.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Scott Murray <scott@spiteful.org>
Cc: Dan Zink <dan.zink@hpe.com>
Cc: Prarit Bhargava <prarit@redhat.com>