ASoC: wm8731: add rates constraints

Depending on the mclk (or crystal) selected, the wm8731 codec have some
constraints on its data sampling rates:
e.g. with a 12.288MHz or 18.432MHz crystal, the authorized rates are
8KHz, 32KHz, 48KHz and 96KHz.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>