dt-bindings: milbeaut-m10v-hdmac: Add Socionext Milbeaut HDMAC bindings

Document the devicetree bindings for Socionext Milbeaut HDMAC
controller. Controller has upto 8 floating channels, that need
a predefined slave-id to work from a set of slaves.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Link: https://lore.kernel.org/r/20191015033350.14866-1-jassisinghbrar@gmail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/milbeaut-m10v-hdmac.txt

+* Milbeaut AHB DMA Controller
+
+Milbeaut AHB DMA controller has transfer capability below.
+ - device to memory transfer
+ - memory to device transfer
+
+Required property:
+- compatible:       Should be  "socionext,milbeaut-m10v-hdmac"
+- reg:              Should contain DMA registers location and length.
+- interrupts:       Should contain all of the per-channel DMA interrupts.
+                     Number of channels is configurable - 2, 4 or 8, so
+                     the number of interrupts specified should be {2,4,8}.
+- #dma-cells:       Should be 1. Specify the ID of the slave.
+- clocks:           Phandle to the clock used by the HDMAC module.
+
+
+Example:
+
+	hdmac1: dma-controller@1e110000 {
+		compatible = "socionext,milbeaut-m10v-hdmac";
+		reg = <0x1e110000 0x10000>;
+		interrupts = <0 132 4>,
+			     <0 133 4>,
+			     <0 134 4>,
+			     <0 135 4>,
+			     <0 136 4>,
+			     <0 137 4>,
+			     <0 138 4>,
+			     <0 139 4>;
+		#dma-cells = <1>;
+		clocks = <&dummy_clk>;
+	};