[ALSA] ASoC codecs: WM8731 support

This patch adds ASoC support for the WM8731 codec.
Supported features:-
 o Capture/Playback/Sidetone/Bypass.
 o 16 & 24 bit audio.
 o 8k - 96k sample rates.
 o DAPM.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Liam Girdwood <liam.girdwood@wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>