dt-bindings: Document the STM32 DMAMUX bindings

This patch adds the documentation of device tree bindings for the STM32
DMAMUX.
Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/devicetree/bindings/dma/stm32-dmamux.txt

+STM32 DMA MUX (DMA request router)
+
+Required properties:
+- compatible:	"st,stm32h7-dmamux"
+- reg:		Memory map for accessing module
+- #dma-cells:	Should be set to <3>.
+		First parameter is request line number.
+		Second is DMA channel configuration
+		Third is Fifo threshold
+		For more details about the three cells, please see
+		stm32-dma.txt documentation binding file
+- dma-masters:	Phandle pointing to the DMA controllers.
+		Several controllers are allowed. Only "st,stm32-dma" DMA
+		compatible are supported.
+
+Optional properties:
+- dma-channels : Number of DMA requests supported.
+- dma-requests : Number of DMAMUX requests supported.
+- resets: Reference to a reset controller asserting the DMA controller
+- clocks: Input clock of the DMAMUX instance.
+
+Example:
+
+/* DMA controller 1 */
+dma1: dma-controller@40020000 {
+	compatible = "st,stm32-dma";
+	reg = <0x40020000 0x400>;
+	interrupts = <11>,
+		     <12>,
+		     <13>,
+		     <14>,
+		     <15>,
+		     <16>,
+		     <17>,
+		     <47>;
+	clocks = <&timer_clk>;
+	#dma-cells = <4>;
+	st,mem2mem;
+	resets = <&rcc 150>;
+	dma-channels = <8>;
+	dma-requests = <8>;
+};
+
+/* DMA controller 1 */
+dma2: dma@40020400 {
+	compatible = "st,stm32-dma";
+	reg = <0x40020400 0x400>;
+	interrupts = <56>,
+		     <57>,
+		     <58>,
+		     <59>,
+		     <60>,
+		     <68>,
+		     <69>,
+		     <70>;
+	clocks = <&timer_clk>;
+	#dma-cells = <4>;
+	st,mem2mem;
+	resets = <&rcc 150>;
+	dma-channels = <8>;
+	dma-requests = <8>;
+};
+
+/* DMA mux */
+dmamux1: dma-router@40020800 {
+	compatible = "st,stm32h7-dmamux";
+	reg = <0x40020800 0x3c>;
+	#dma-cells = <3>;
+	dma-requests = <128>;
+	dma-channels = <16>;
+	dma-masters = <&dma1 &dma2>;
+	clocks = <&timer_clk>;
+};
+
+/* DMA client */
+usart1: serial@40011000 {
+	compatible = "st,stm32-usart", "st,stm32-uart";
+	reg = <0x40011000 0x400>;
+	interrupts = <37>;
+	clocks = <&timer_clk>;
+	dmas = <&dmamux1 41 0x414 0>,
+	       <&dmamux1 42 0x414 0>;
+	dma-names = "rx", "tx";
+};