PCI PM: Introduce pci_prepare_to_sleep and pci_back_from_sleep

Introduce functions pci_prepare_to_sleep() and pci_back_from_sleep(),
to be used by the PCI drivers that want to place their devices into
the lowest power state appropiate for them (PCI_D3hot, if the device
is not supposed to wake up the system, or the deepest state from
which the wake-up is possible, otherwise) while the system is being
prepared to go into a sleeping state and to put them back into D0
during the subsequent transition to the working state.

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