ACPI: x86: s2idle: Add a function to get LPS0 constraint for a device

Other parts of the kernel may use LPS0 constraints information to make
decisions on what power state to put a device into.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
[ rjw: Rewrite kerneldoc, rearrange if () statement, edit subject and
  changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>