doc: dt: dma: add bindings for lpc1850-dmamux

Add device tree bindings documentation for the
lpc1850-dmamux DMA router.
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/devicetree/bindings/dma/lpc1850-dmamux.txt

+NXP LPC18xx/43xx DMA MUX (DMA request router)
+
+Required properties:
+- compatible:	"nxp,lpc1850-dmamux"
+- reg:		Memory map for accessing module
+- #dma-cells:	Should be set to <3>.
+		* 1st cell contain the master dma request signal
+		* 2nd cell contain the mux value (0-3) for the peripheral
+		* 3rd cell contain either 1 or 2 depending on the AHB
+		  master used.
+- dma-requests:	Number of DMA requests for the mux
+- dma-masters:	phandle pointing to the DMA controller
+
+The DMA controller node need to have the following poroperties:
+- dma-requests:	Number of DMA requests the controller can handle
+
+Example:
+
+dmac: dma@40002000 {
+	compatible = "nxp,lpc1850-gpdma", "arm,pl080", "arm,primecell";
+	arm,primecell-periphid = <0x00041080>;
+	reg = <0x40002000 0x1000>;
+	interrupts = <2>;
+	clocks = <&ccu1 CLK_CPU_DMA>;
+	clock-names = "apb_pclk";
+	#dma-cells = <2>;
+	dma-channels = <8>;
+	dma-requests = <16>;
+	lli-bus-interface-ahb1;
+	lli-bus-interface-ahb2;
+	mem-bus-interface-ahb1;
+	mem-bus-interface-ahb2;
+	memcpy-burst-size = <256>;
+	memcpy-bus-width = <32>;
+};
+
+dmamux: dma-mux {
+	compatible = "nxp,lpc1850-dmamux";
+	#dma-cells = <3>;
+	dma-requests = <64>;
+	dma-masters = <&dmac>;
+};
+
+uart0: serial@40081000 {
+	compatible = "nxp,lpc1850-uart", "ns16550a";
+	reg = <0x40081000 0x1000>;
+	reg-shift = <2>;
+	interrupts = <24>;
+	clocks = <&ccu2 CLK_APB0_UART0>, <&ccu1 CLK_CPU_UART0>;
+	clock-names = "uartclk", "reg";
+	dmas = <&dmamux 1 1 2
+		&dmamux 2 1 2>;
+	dma-names = "tx", "rx";
+};