ACPI: Add acpi_pm_device_sleep_state helper routine

Based on the David Brownell's patch at
http://marc.info/?l=linux-acpi&m=117873972806360&w=2
updated by: Rafael J. Wysocki <rjw@sisk.pl>

Add a helper routine returning the lowest power (highest number) ACPI device
power state that given device can be in while the system is in the sleep state
indicated by acpi_target_sleep_state .

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