platform/x86: intel_scu_ipc: Add constants for register offsets

This makes the code more readable. These are taken from intel_pmc_ipc.c
which implements the same thing.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>