ALSA: ac97: add codecs devicetree binding

Add a devicetree binding for codecs. This is especially useful if the
AC97 bitclk clock is provided by the codec, as it has to be described in
the devicetree description for the ac97 bus code to aquire it.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>