ASoC: WM8991: Add initial WM8991 driver

The WM8991 is a highly integrated ultra-low power hi-fi CODEC designed for
handsets rich in multimedia features such as GPS, mobile TV, digital audio
playback and gaming.

This driver was originally written by Graeme Gregory and has been maintained
out of tree by Mark Brown and Dimitris Papastamos.

Signed-off-by: Graeme Gregory <gg@opensource.wolfsonmicro.com>
Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>