ACPI: processor: idle: Only flush cache on entering C3

According to ACPI 6.4, Section 8.2, CPU cache flushing required on
entering the C3 power state.

Avoid flushing the cache on entering other C-states.

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