mfd: wm831x: Add device tree binding document

Add a device tree binding document for the wm831x series of PMICs.
Currently only support for the registering the device and the GPIOs are
actually implemented in the driver.

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