ASoC: soc-core: add .num_platform for dai_link

Current snd_soc_dai_link is starting to use snd_soc_dai_link_component
(= modern) style for Platform, but it is still assuming single Platform
so far. We will need to have multi Platform support in the not far
future.

Currently only simple card is using it as sound card driver,
and other drivers are converted to it from legacy style by
snd_soc_init_platform().
To avoid future problem of multi Platform support, let's add
num_platforms before it is too late.

In the same time, to make it same naming mothed, "platform" should
be "platforms". This patch fixup it too.

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