ASoC: simple-card: tidyup prefix for snd_soc_codec_conf

Current simple-card is handling "prefix" by many ways.
But, it is not useful and readable.
We want to do is that allow having it everywere.
This patch supports it.
It will be overwrote if lower node has it.

sound {
	simple-audio-card,prefix = "xxx"; // initial

	simple-audio-card,dai-link {
		prefix = "xxx"; // overwrite
		cpu {
			...
		};
		codec {
			prefix = "xxx"; // overwrite
		};
	};
};

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