ASoC: mxs: Enable COMPILE_TEST builds

Since DT based boards don't have any dependency on arch/arm enable them
if COMPILE_TEST is enabled.
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/mxs/Kconfig

 menuconfig SND_MXS_SOC
 	tristate "SoC Audio for Freescale MXS CPUs"
-	depends on ARCH_MXS
+	depends on ARCH_MXS || COMPILE_TEST
 	select SND_SOC_GENERIC_DMAENGINE_PCM
 	help
 	  Say Y or M if you want to add support for codecs attached to