ASoC: davinci-evm: Add named clock reference to DT bindings

The referenced clock is used to get codec clock rate and the clock is
disabled and enabled in startup and shutdown snd_soc_ops call
backs. The change is also documented in DT bindigs document.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
cc: bcousson@baylibre.com
Signed-off-by: Mark Brown <broonie@linaro.org>