mfd: wm831x: Add basic device tree binding

Add the basic ability to register the device through device tree, more
work is needed to get each individual sub-driver functioning correctly
but this is enough to get the device to probe from device tree.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>