ASoC: rsnd: move "renesas,rsrc-card" to "simple-scu-audio-card" on Document

"renesas,rsrc-card" is exchanged to "simple-scu-card".
Let's update Document

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