mfd: Add support for WM8320 PMICs

The WM8320 is an integrated power management subsystem providing
voltage regulators, RTC, watchdog and other functionality. The
WM8320 is derived from the WM831x and therefore shares most of
the driver code with the WM831x.

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