ASoC: audio-graph-card: tidyup prefix for snd_soc_codec_conf

Current audio-graph-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 {
		prefix = "xxx"; // initial
	};

	codec {
		audio-graph-card,prefix = "xxx"; // overwrite
		ports {
			prefix = "xxx"; // overwrite
			port {
				prefix = "xxx"; // overwrite
			};
		};
	};

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