ASoC: wm8997: Initial CODEC driver

The wm8997 is a compact, high-performance audio hub CODEC with SLIMbus
interfacing, for smartphones, tablets and other portable audio devices
based on the Arizona platform.

This patch adds the wm8997 CODEC driver.

[Fixed some interface churn from bitrot due to the patch not going via
the MFD tree as expected -- broonie]

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>