This patch adds MIX (Mixer) initial support for rsnd driver.
It is assuming that this MIX is used via DPCM.

This is sample code for playback.

	CPU0  : [MEM] -> [SRC1] -> [CTU02] -+
					    |
					    +-> [MIX0] -> [DVC0] -> [SSI0]
	                                    |
	CPU1  : [MEM] -> [SRC2] -> [CTU03] -+

	sound {
		compatible = "renesas,rsrc-card";

		...

		cpu@0 {
			sound-dai = <&rcar_sound 0>;
		};

		cpu@1 {
			sound-dai = <&rcar_sound 1>;
		};

		codec {
			...
		};
	};

	rcar_sound {

		...

		rcar_sound,dai {
			dai0 {
				playback = <&src1 &ctu02 &mix0 &dvc0 &ssi0>;
			};
			dai1 {
				playback = <&src2 &ctu03 &mix0 &dvc0 &ssi0>;
			};
		};
	};
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>