platform/x86: intel_scu_ipc: Move legacy SCU IPC API to a separate header

In preparation for introducing a new API for SCU IPC, move the legacy
API and constants to a separate header that is is subject to be removed
eventually.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>