ACPI: Introduce acpi_bus_power_manageable function

Introduce function acpi_bus_power_manageable() allowing other
(dependent) subsystems to check if ACPI is able to power manage given
device.  This may be useful, for example, for PCI device power
management.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>