ACPI / processor: Set P_LVL{2,3} idle state descriptions

The ACPI idle driver will fallback to using the legacy P_LVL* SystemIO
method of entering C-states if the _CST method is disabled and P_BLK is
defined. However, in this case the C2 and C3 states won't have a
description set, so the user will see "<null>" when reading the
description from sysfs.

Give each of these states a description.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>