ASoC: Intel: Fix build for sst-dsp.c on PPC architecture

Disable build on non X86 architectures except for compile testing. This fixes
the following build errors on PPC and adds an option for testing the build on other
architectures as suggested by Mark Brown :-

sound/soc/intel/sst-dsp.c: In function 'sst_dsp_outbox_write':
sound/soc/intel/sst-dsp.c:218:2: error: implicit declaration of function 'memcpy_toio' [-Werror=implicit-function-declaration]
  memcpy_toio(sst->mailbox.out_base, message, bytes);
  ^
sound/soc/intel/sst-dsp.c: In function 'sst_dsp_outbox_read':
sound/soc/intel/sst-dsp.c:231:2: error: implicit declaration of function 'memcpy_fromio' [-Werror=implicit-function-declaration]
  memcpy_fromio(message, sst->mailbox.out_base, bytes);
  ^

Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>