ACPI: Implement the set_target() callback from pm_ops

In the future some drivers may need to use ACPI to determine the low power
states in which to place their devices, but to provide the drivers with this
information the ACPI core needs to know what sleep state the system is going to
enter.  Namely, the device's state should not be too high power for given system
sleep state and, if the device is supposed to be able to wake up the system, its
state should not be too low power for the wake up to be possible).  For this
purpose, the ACPI core needs to implement the set_target() method in 'struct
pm_ops' and store the target system sleep state passed by the PM core in a
variable.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Len Brown <len.brown@intel.com>