ACPI: x86: s2idle: Add for_each_lpi_constraint() helper

We have one existing and one coming user of this macro.
Introduce a helper.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>