PCI: Introduce platform_pci_power_manageable function

Introduce function pointer platform_pci_power_manageable to be used
by the platform-related code to point to a function allowing us to
check if given device is power manageable by the platform.

Introduce acpi_pci_power_manageable() playing that role for ACPI.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>