dt-bindings: mailbox: mtk,adsp-mbox: add mtk adsp-mbox document

This patch adds document for mediatek adsp mbox
Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/mtk,adsp-mbox.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/mtk,adsp-mbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Mediatek ADSP mailbox
+
+maintainers:
+  - Allen-KH Cheng <Allen-KH.Cheng@mediatek.com>
+
+description: |
+  The MTK ADSP mailbox Inter-Processor Communication (IPC) enables the SoC
+  to ommunicate with ADSP by passing messages through two mailbox channels.
+  The MTK ADSP mailbox IPC also provides the ability for one processor to
+  signal the other processor using interrupts.
+
+properties:
+  compatible:
+    items:
+      - const: mediatek,mt8195-adsp-mbox
+
+  "#mbox-cells":
+    const: 0
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - "#mbox-cells"
+  - reg
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/interrupt-controller/irq.h>
+
+    adsp_mailbox0:mailbox@10816000 {
+        compatible = "mediatek,mt8195-adsp-mbox";
+        #mbox-cells = <0>;
+        reg = <0x10816000 0x1000>;
+        interrupts = <GIC_SPI 702 IRQ_TYPE_LEVEL_HIGH 0>;
+    };