ASoC: ep93xx: get rid of ep93xx-pcm-audio struct device

Modify the ep93xx PCM driver so that it's a utility library that can be
registered on each DAI, rather than a separate struct device. This is
more in line with how many recent DT-converted platforms operate, and
avoids the need for yet another struct device.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>