ASoC: convert platform explanation to component

commit ef050bec ("ASoC: Remove platform code now everything is
componentised") removed platform code, but it didn't care
about platform documentation.
This patch convert platform explanation to component

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>