Documentation: Add the AB8500 CODEC device to the MFD AB8500 doc

Here we document the AB8500 Audio CODEC in the existing MFD AB8500 document.

CC: alsa-devel@alsa-project.org
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>