ASoC: simple-scu-card: cleanup documentation

simple-scu-card is almost same as simple-card. This is already explained
in document. But simple-card and simple-scu-card both has same
explanation for same property. This patch forward explantion to
simple-card if possible to avoid duplication.

This patch also cleanup DT binding example which is not good matching
to simple-scu-card.

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