x86, mrst: add platform support for MSIC MFD driver

The MSIC MFD driver creates platform devices for MSIC device drivers so we
don't need to create them in platform code anymore.

This patch adds a new runtime check which determines whether we are running
on a Medfield platform and enables the MSIC MFD driver accordingly.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>