ASoC: Export devm_snd_soc_register_platform()

devm_snd_soc_register_platform() is used in drivers which can be build as
modules, so it needs to be exported to avoid linkers errors like:

	ERROR: "devm_snd_soc_register_platform" [sound/soc/omap/snd-soc-omap.ko] undefined!
	ERROR: "devm_snd_soc_register_platform" [sound/soc/davinci/snd-soc-davinci.ko] undefined!

Fixes: 8931bf62 ("ASoC: Add resource managed snd_soc_register_platform()")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>