ACPI / hotplug / PCI: Consolidate ACPIPHP with ACPI core hotplug

The ACPI-based PCI hotplug (ACPIPHP) code currently attaches its
hotplug context objects directly to ACPI namespace nodes representing
hotplug devices.  However, after recent changes causing struct
acpi_device to be created for every namespace node representing a
device (regardless of its status), that is not necessary any more.
Moreover, it's vulnerable to the theoretical issue that the ACPI
handle passed in the context between handle_hotplug_event() and
hotplug_event_work() may become invalid in the meantime (as a
result of a concurrent table unload).

In principle, this issue might be addressed by adding a non-empty
release handler for ACPIPHP hotplug context objects analogous to
acpi_scan_drop_device(), but that would duplicate the code in that
function and in acpi_device_del_work_fn().  For this reason, it's
better to modify ACPIPHP to attach its device hotplug contexts to
struct device objects representing hotplug devices and make it
use acpi_hotplug_notify_cb() as its notify handler.  At the same
time, acpi_device_hotplug() can be modified to dispatch the new
.hp.event() callback pointing to acpiphp_hotplug_event() from ACPI
device objects associated with PCI devices or use the generic
ACPI device hotplug code for device objects with matching scan
handlers.

This allows the existing code duplication between ACPIPHP and the
ACPI core to be reduced too and makes further ACPI-based device
hotplug consolidation possible.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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