ASoC: Add WM8776 CODEC driver

The WM8776 is a high performance, stereo audio CODEC with five channel
input selector. The WM8776 is ideal for surround sound processing
applications for home hi-fi, DVD-RW and other audio visual equipment.

This driver implements support for most WM8776 features - currently the
ADC automatic level control/limiter functionality is omitted.

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