ACPI / processor: Set default C1 idle state description

The ACPI idle driver will default to ACPI_CSTATE_HALT for C1 if a _CST
object for C1 is not defined. However, the description will not be set,
so users will see "<null>" when reading the description from sysfs.

Set the C1 state description when defaulting to ACPI_CSTATE_HALT.

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