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";
};