platform/x86: ISST: Allow additional core-power mailbox commands

To discover core-power capability, some new mailbox commands are added. Allow
those commands to execute.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>