mfd: Add initial WM8994 support

The WM8994 is a highly integrated ultra low power audio hub CODEC.
Since it includes on-board regulators and GPIOs it is represented
as a multi-function device, though the overwhelming majority of
the functionality is provided by the ASoC CODEC driver.

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