[ALSA] ASoC WM8731 support for 32k @ 12MHz sysclk

This patch adds support for 32k audio on the WM8731 when running from a
12MHz system clock.

Signed-off-by: Frank Mandarino <fmandarino@endrelia.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>