ASoC: imx-sgtl5000: use of_node to match cpu dai

Since imx-sgtl5000 is only used on DT platform, it makes more sense to
use cpu_of_node rather than cpu_dai_name to match cpu dai.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>