mfd: Add WM8321 support

The WM8321 is a PMIC for low power, high performance applications. From a
software point of view the device is identical to the WM8320, all the
differences between the two devices are visible only in hardware.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>