dt-bindings: mailbox: Add support for bcm74110

Add devicetree YAML binding for brcmstb bcm74110 mailbox used
for communicating with a co-processor.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Justin Chen <justin.chen@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Documentation/devicetree/bindings/mailbox/brcm,bcm74110-mbox.yaml

@@ -0,0 +1,64 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/brcm,bcm74110-mbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom BCM74110 Mailbox
+
+maintainers:
+  - Justin Chen <justin.chen@broadcom.com>
+  - Florian Fainelli <florian.fainelli@broadcom.com>
+
+description: Broadcom mailbox hardware first introduced with 74110
+
+properties:
+  compatible:
+    enum:
+      - brcm,bcm74110-mbox
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    items:
+      - description: RX doorbell and watermark interrupts
+      - description: TX doorbell and watermark interrupts
+
+  "#mbox-cells":
+    const: 2
+    description:
+      The first cell is channel type and second cell is shared memory slot
+
+  brcm,rx:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: RX Mailbox number
+
+  brcm,tx:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: TX Mailbox number
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - "#mbox-cells"
+  - brcm,rx
+  - brcm,tx
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/irq.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+
+    mailbox@a552000 {
+        compatible = "brcm,bcm74110-mbox";
+        reg = <0xa552000 0x1104>;
+        interrupts = <GIC_SPI 0x67 IRQ_TYPE_LEVEL_HIGH>,
+                     <GIC_SPI 0x66 IRQ_TYPE_LEVEL_HIGH>;
+        #mbox-cells = <0x2>;
+        brcm,rx = <0x7>;
+        brcm,tx = <0x6>;
+    };