platform:x86: add Intel P-Unit mailbox IPC driver

This driver provides support for P-Unit mailbox IPC on Intel platforms.
The heart of the P-Unit is the Foxton microcontroller and its firmware,
which provide mailbox interface for power management usage.

Signed-off-by: Qipeng Zha <qipeng.zha@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>