ASoC: Add resource managed snd_soc_register_platform()

Simplify error handling and remove repetitive (and rarely executed) code
for unregistration by providing a devm_snd_soc_register_platform()
platform.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>