ASoC: simple-card: card name can be option

snd_card.name is now option on DT case.
non-DT case can follow same style,
and it is understandable from platform point of view.

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