ACPI / PCI: Make PCI root driver use struct acpi_scan_handler

Make the ACPI PCI root bridge driver use struct acpi_scan_handler
for representing the object used to enumerate the PCI busses under
PCI host bridges found in the ACPI namespace (and to tear down data
structures representing the bus and devices on it before
unregistering the host bridges' ACPI device nodes).

This simplifies the code slightly and reduces the kernel's memory
footprint by avoiding the registration of a struct device_driver
object with the driver core and creation of its sysfs directory
which is unnecessary.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Toshi Kani <toshi.kani@hp.com>