intel_idle: mark cpu id array as __initconst

The CPU ids are only tested in intel_idle_probe() which is itself an
__init function. For the MODULE_DEVICE_TABLE() file2alias doesn't care
about the section, just about the symbol name. So it's safe to mark
the cpu id array as  __initconst so its memory can be released after
initialization is done.

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