ASoC: soc-core: support dai_link with platforms_num != 1

Add support platforms_num != 1 in dai_link. Initially, the main purpose of
this change was to make the platform optional in the dai_link, instead of
inserting the dummy platform driver.

This particular case had just been solved by Kuninori Morimoto with
commit 1d768989 ("ASoC: soc-core: allow no Platform on dai_link").

However, this change may still be useful for those who need multiple
platform components on a single dai_link (it solves one of the FIXME
note in soc-core)

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