dt-bindings: dmaengine: sf-pdma: add bindins for SiFive PDMA

Add DT bindings document for Platform DMA(PDMA) driver of board,
HiFive Unleashed Rev A00.
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Signed-off-by: Green Wan <green.wan@sifive.com>
Link: https://lore.kernel.org/r/20191107084955.7580-2-green.wan@sifive.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/sifive,fu540-c000-pdma.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/dma/sifive,fu540-c000-pdma.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: SiFive Unleashed Rev C000 Platform DMA
+
+maintainers:
+  - Green Wan <green.wan@sifive.com>
+  - Palmer Debbelt <palmer@sifive.com>
+  - Paul Walmsley <paul.walmsley@sifive.com>
+
+description: |
+  Platform DMA is a DMA engine of SiFive Unleashed. It supports 4
+  channels. Each channel has 2 interrupts. One is for DMA done and
+  the other is for DME error.
+
+  In different SoC, DMA could be attached to different IRQ line.
+  DT file need to be changed to meet the difference. For technical
+  doc,
+
+  https://static.dev.sifive.com/FU540-C000-v1.0.pdf
+
+properties:
+  compatible:
+    items:
+      - const: sifive,fu540-c000-pdma
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    minItems: 1
+    maxItems: 8
+
+  '#dma-cells':
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - '#dma-cells'
+
+examples:
+  - |
+    dma@3000000 {
+      compatible = "sifive,fu540-c000-pdma";
+      reg = <0x0 0x3000000 0x0 0x8000>;
+      interrupts = <23 24 25 26 27 28 29 30>;
+      #dma-cells = <1>;
+    };
+
+...