ACPI: processor: idle: Allow probing on platforms with one ACPI C-state

It is possible for ACPI _CST to return only one ACPI C-state, for
example, when deep cstate disabled in the BIOS.

And it is better for the acpi_idle driver to probe succesfully in
this case as well for consistency.

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