powercap / RAPL: mark rapl_ids array as __initconst

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

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>