x86: Add SCU IPC driver for Intel MID platforms

Intel MID platforms have few microcontrollers inside SoC, one of them
is so called System Controller Unit (SCU).

Here is the driver to communicate with microcontroller.

Signed-off-by: Vincent Tinelli <vincent.tinelli@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Simon Glass <sjg@chromium.org>