ASoC: Intel: Add help text for SND_SOC_INTEL_SST_TOPLEVEL

Symbol SND_SOC_INTEL_SST_TOPLEVEL is user selectable so add the
help text for this symbol.

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>