ACPI: Add stubs for (un)register_acpi_bus_type

It's unreasonable to have CONFIG_ACPI for these in drivers, so add some
stub functions.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>