mfd: wm97xx-core: core support for wm97xx Codec

The WM9705, WM9712 and WM9713 are highly integrated codecs, with an
audio codec, DAC and ADC, GPIO unit and a touchscreen interface.

Historically the support was spread across drivers/input/touchscreen and
sound/soc/codecs. The sharing was done through ac97 bus sharing. This
model will not withstand the new AC97 bus model, where codecs are
discovered on runtime.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>