dt-bindings: mailbox: Add the Spreadtrum mailbox documentation

Add the Spreadtrum mailbox documentation.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@unisoc.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/sprd-mailbox.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/mailbox/sprd-mailbox.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Spreadtrum mailbox controller bindings
+
+maintainers:
+  - Orson Zhai <orsonzhai@gmail.com>
+  - Baolin Wang <baolin.wang7@gmail.com>
+  - Chunyan Zhang <zhang.lyra@gmail.com>
+
+properties:
+  compatible:
+    enum:
+      - sprd,sc9860-mailbox
+
+  reg:
+    items:
+      - description: inbox registers' base address
+      - description: outbox registers' base address
+
+  interrupts:
+    items:
+      - description: inbox interrupt
+      - description: outbox interrupt
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: enable
+
+  "#mbox-cells":
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - "#mbox-cells"
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    mailbox: mailbox@400a0000 {
+      compatible = "sprd,sc9860-mailbox";
+      reg = <0 0x400a0000 0 0x8000>, <0 0x400a8000 0 0x8000>;
+      #mbox-cells = <1>;
+      clock-names = "enable";
+      clocks = <&aon_gate 53>;
+      interrupts = <GIC_SPI 28 IRQ_TYPE_LEVEL_HIGH>, <GIC_SPI 29 IRQ_TYPE_LEVEL_HIGH>;
+    };
+...