ASoC: simple-card: platform also uses asoc_simple_card_sub_parse_of()

In current simple-card, platform is handled as special case,
but, the code is not readable.
This patch makes platform to use asoc_simple_card_sub_parse_of()

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