ACPI / hotplug / PCI: Register all devices under the given bridge

Rework register_slot() to create a struct acpiphp_func object for
every function it is called for and to create acpiphp slots for all
of them.  Although acpiphp_register_hotplug_slot() is only called for
the slots whose functions are identified as "ejectable", so that user
space can manipulate them, the ACPIPHP notify handler,
handle_hotplug_event(), is now installed for all of the registered
functions (that aren't dock stations) and hotplug events may be
handled for all of them.

As a result, essentially, all PCI bridges represented by objects in
the ACPI namespace are now going to be "hotplug" bridges and that may
affect resources allocation in general, although it shouldn't lead to
problems.

This allows the code to be simplified substantially and addresses
the problem where bus check or device check notifications for some
PCI bridges or devices are not handled, because those devices are
not recognized as "ejectable" or there appear to be no "ejectable"
devices under those bridges.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>