ASoC: s6000: Allow to build when COMPILE_TEST is enabled

Most of the ASoC s6000 code is architecture independent. This patch makes it
possible to select the platform when COMPILE_TEST is enabled.

The only architecture dependent code is the PCM driver which will still only be
selected if XTENSA_VARIANT_S6000 is enabled.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>