ASoC: Add WM8974 CODEC driver

The WM8974 is a low power, high quality mono CODEC designed for portable
applications such as digital still cameras or digital voice recorders.

This driver was originally written by Graeme Gregory and Liam Girdwood
and has since been maintained by myself with some updates contributed by
Brett Saunders and Javier Martin.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>