gpio: wm831x: Add basic device tree support

Now the wm831x-core has basic DT support we can update this driver to
allow use of the GPIOs within a device tree system.

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