dt-bindings: dmaengine: Add UniPhier external DMA controller bindings

Add devicetree binding documentation for external DMA controller
implemented on Socionext UniPhier SOCs.
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1582271550-3403-2-git-send-email-hayashi.kunihiko@socionext.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/socionext,uniphier-xdmac.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/dma/socionext,uniphier-xdmac.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Socionext UniPhier external DMA controller
+
+description: |
+  This describes the devicetree bindings for an external DMA engine to perform
+  memory-to-memory or peripheral-to-memory data transfer capable of supporting
+  16 channels, implemented in Socionext UniPhier SoCs.
+
+maintainers:
+  - Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
+
+allOf:
+  - $ref: "dma-controller.yaml#"
+
+properties:
+  compatible:
+    const: socionext,uniphier-xdmac
+
+  reg:
+    items:
+      - description: XDMAC base register region (offset and length)
+      - description: XDMAC extension register region (offset and length)
+
+  interrupts:
+    maxItems: 1
+
+  "#dma-cells":
+    const: 2
+    description: |
+      DMA request from clients consists of 2 cells:
+        1. Channel index
+        2. Transfer request factor number, If no transfer factor, use 0.
+           The number is SoC-specific, and this should be specified with
+           relation to the device to use the DMA controller.
+
+  dma-channels:
+    minimum: 1
+    maximum: 16
+
+additionalProperties: false
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - "#dma-cells"
+
+examples:
+  - |
+    xdmac: dma-controller@5fc10000 {
+        compatible = "socionext,uniphier-xdmac";
+        reg = <0x5fc10000 0x1000>, <0x5fc20000 0x800>;
+        interrupts = <0 188 4>;
+        #dma-cells = <2>;
+        dma-channels = <16>;
+    };
+
+...