ASoC: sst_platform: fix the dsp driver interface

lower level drivers typically register with upper layers.
So fix by exporting symbols from sst_platform driver for dsp driver to
register to sst platform driver

Now this driver doesnt depend on sst driver, so remove the dependency
and the header files

Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>