PCI: acpiphp: Do not use ACPI PCI subdriver mechanism

PCI: acpiphp: Do not use ACPI PCI subdriver mechanism

Previously the acpiphp driver registered itself as an ACPI PCI subdriver,
so its callbacks were invoked when creating/destroying PCI root
buses to manage ACPI-based PCI hotplug slots.  But it doesn't handle
P2P bridge hotplug events, so it will cause strange behaviour if there
are hotplug slots associated with a hot-removed P2P bridge.

This patch fixes this issue by:
1) Directly hooking into PCI core to update hotplug slot devices when
   creating/destroying PCI buses through:
	pci_{add|remove}_bus() -> acpi_pci_{add|remove}_bus()
2) Getting rid of unused ACPI PCI subdriver-related code

It also cleans up unused code in the acpiphp driver.

[bhelgaas: keep acpi_pci_add_bus() stub for CONFIG_ACPI=n]
Signed-off-by: Jiang Liu <[email protected]>
Signed-off-by: Yijing Wang <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Yinghai Lu <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Toshi Kani <[email protected]>