ASoC: kirkwood: armada-370-db: don't select unnecessary Platform

ALSA SoC is now supporting "no Platform". Sound card doesn't need to
select "CPU component" as "Platform" anymore if it doesn't need
special Platform.
This patch removes such settings.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>