mfd: wm8994: Add device ID data to WM8994 OF device IDs

We can actually read this back from the device but we use this when
registered using standard I2C board data registration so make sure
it's there for OF too.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>