mfd: Add basic device tree binding for wm8994

Add a placeholder device tree binding for the wm8994 driver. At present
the binding is essentially null as none of the platform data is supported,
and at least some of that will depend on the pending regulator bindings.

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