platform/x86: intel_mid_powerbtn: Use SCU IPC directly

On older Intel MID platforms is using SCU IPC library beneath MSIC
calls.

To make access unified between old and new platforms use SCU IPC library
directly. It's safe since serialization is done in the library.

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