On some systems, in order to get to the deepest low-power state of
the platform (which may be necessary to save significant enough
amounts of energy while suspended to idle. for example), devices on
the PCI bus exposed by the VMD driver need to be power-managed via
ACPI.  However, the layout of the ACPI namespace below the VMD
controller device object does not reflect the layout of the PCI bus
under the VMD host bridge, so in order to identify the ACPI companion
objects for the devices on that bus, it is necessary to use a special
_ADR encoding on the ACPI side.  In other words, acpi_pci_find_companion()
does not work for these devices, so it needs to be amended with a
special lookup logic specific to the VMD bus.

Address this issue by allowing the VMD driver to temporarily install
an ACPI companion lookup hook containing the code matching the devices
on the VMD PCI bus with the corresponding objects in the ACPI
namespace.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Jon Derrick <jonathan.derrick@intel.com>