ASoC: fsl_mqs: add DT binding documentation

Add the DT binding documentation for NXP MQS driver
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/65e1f035aea2951aacda54aa3a751bc244f72f6a.1568367274.git.shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl,mqs.txt

+fsl,mqs audio CODEC
+
+Required properties:
+  - compatible : Must contain one of "fsl,imx6sx-mqs", "fsl,codec-mqs"
+		"fsl,imx8qm-mqs", "fsl,imx8qxp-mqs".
+  - clocks : A list of phandles + clock-specifiers, one for each entry in
+	     clock-names
+  - clock-names : "mclk" - must required.
+		  "core" - required if compatible is "fsl,imx8qm-mqs", it
+		           is for register access.
+  - gpr : A phandle of General Purpose Registers in IOMUX Controller.
+	  Required if compatible is "fsl,imx6sx-mqs".
+
+Required if compatible is "fsl,imx8qm-mqs":
+  - power-domains: A phandle of PM domain provider node.
+  - reg: Offset and length of the register set for the device.
+
+Example:
+
+mqs: mqs {
+	compatible = "fsl,imx6sx-mqs";
+	gpr = <&gpr>;
+	clocks = <&clks IMX6SX_CLK_SAI1>;
+	clock-names = "mclk";
+	status = "disabled";
+};
+
+mqs: mqs@59850000 {
+	compatible = "fsl,imx8qm-mqs";
+	reg = <0x59850000 0x10000>;
+	clocks = <&clk IMX8QM_AUD_MQS_IPG>,
+		 <&clk IMX8QM_AUD_MQS_HMCLK>;
+	clock-names = "core", "mclk";
+	power-domains = <&pd_mqs0>;
+	status = "disabled";
+};