ASoC: audio-graph-card: tidyup convert_rate/channel method

Current audio-graph-card is handling "convert_rate/channel"
by many ways. But, it is not useful and readable.
We want to do is that allow having it everywere.
This patch support it.
It will be overwrote if lower node has it.

sound {
	convert-channels = <xxx>; // initial
};

codec {
	audio-graph-card,convert-channels = <xxx>; // overwrite
	ports {
		convert_channels = <xxx>; // overwrite
		port {
			convert_channels = <xxx>; // overwrite
			endpoint {
				convert_channels = <xxx>; // overwrite
			};
		};
	};
};

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