ASoC: rsnd: add MIX (Mixer) support

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>