mfd: wm8994: Add some OF properties

Add properties for some of the more important bits of platform data and
fill out the binding document.

Not all of the current platform data is suitable for the sort of fixed
configuration that is done using DT, some of it should have runtime
mechanisms added instead and some is unlikely to ever be used in practical
systems.

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