ASoC: Add WM8711 CODEC driver

The WM8711 or WM8711L (WM8711/L) is a low power stereo DAC with an
integrated headphone driver. The WM8711/L is designed specifically for
portable MP3 audio and speech players. The WM8711/L is also ideal for
MD, CD machines and DAT players.

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