ACPI / PM: Use string "D3cold" to represent ACPI_STATE_D3_COLD

Make acpi_power_state_string() return "D3cold" as the string
representation of ACPI power state D3cold instead of "D3" returned
currently, which is confusing.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>