ASoC: smdk_wm8994: Convert to use snd_soc_register_card()

Current method for machine driver to register with the ASoC core is to use
snd_soc_register_card() instead of creating a "soc-audio" platform device.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>