ALSA: ASoC: Convert wm8731 to a new-style i2c driver

Convert the wm8731 codec driver to the new (standard) device driver
binding model.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>