ASoC: zx-tdm: add bindings doc for zte's tdm controller

This patch adds dt-binding documentation for zte's tdm controller.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/zte,tdm.txt

+ZTE TDM DAI driver
+
+Required properties:
+
+- compatible : should be one of the following.
+       * zte,zx296718-tdm
+- reg : physical base address of the controller and length of memory mapped
+    region.
+- clocks : Pairs of phandle and specifier referencing the controller's clocks.
+- clock-names: "wclk" for the wclk.
+               "pclk" for the pclk.
+-#clock-cells: should be 1.
+- zte,tdm-dma-sysctrl : Reference to the sysctrl controller controlling
+    the dma. includes:
+	phandle of sysctrl.
+	register offset in sysctrl for control dma.
+	mask of the register that be written to sysctrl.
+
+Example:
+
+	tdm: tdm@1487000 {
+		compatible = "zte,zx296718-tdm";
+		reg = <0x01487000 0x1000>;
+		clocks = <&audiocrm AUDIO_TDM_WCLK>, <&audiocrm AUDIO_TDM_PCLK>;
+		clock-names = "wclk", "pclk";
+		#clock-cells = <1>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&tdm_global_pin>;
+		zte,tdm-dma-sysctrl = <&sysctrl 0x10c 4>;
+	};