ASoC: soc-core: add comment for for_each_xxx

soc-core has many for_each_xxx, but it is a little bit
difficult to know which list is relead to which for_each_xxx.
This patch adds missing comment for it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87lfw5ahpj.wl-kuninori.morimoto.gx@renesas.com


Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>