mfd: Add Intel MSIC driver

Add support for Intel MSIC chip found on Intel Medfield platforms. This
chip embeds several subdevices: audio, ADC, GPIO, power button, etc. The
driver creates platform device for each subdevice.

We also provide an MSIC register access API which should replace the more
generic SCU IPC interface currently used. Existing drivers can choose
whether they convert to this new API or stick with the SCU IPC interface.

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