sound: ASoC: Add WM8971 CODEC driver

The WM8971 is a low power, high quality stereo codec designed for
portable digital audio applications.

This driver was originally written by Kenneth Kiraly.  While out of tree
it has had updates to reflect current kernel APIs and coding standards
from Graeme Gregory and Mark Brown.

Signed-off-by: Kenneth Kiraly <kiraly@lab126.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Graeme Gregory <gg@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>