ACPI: PM: Avoid CPU cache flush when entering S4

According to ACPI 6.4, Section 16.2, the CPU cache flushing is
required on entering to S1, S2, and S3.

No need to flush the caches during hibernation (S4).

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>