dt-bindings: mailbox: qcom: Add CPUCP mailbox controller bindings

Add devicetree binding for CPUSS Control Processor (CPUCP) mailbox
controller.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sibi Sankar <quic_sibis@quicinc.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Documentation/devicetree/bindings/mailbox/qcom,cpucp-mbox.yaml

+# SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/qcom,cpucp-mbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Qualcomm Technologies, Inc. CPUCP Mailbox Controller
+
+maintainers:
+  - Sibi Sankar <quic_sibis@quicinc.com>
+
+description:
+  The CPUSS Control Processor (CPUCP) mailbox controller enables communication
+  between AP and CPUCP by acting as a doorbell between them.
+
+properties:
+  compatible:
+    items:
+      - const: qcom,x1e80100-cpucp-mbox
+
+  reg:
+    items:
+      - description: CPUCP rx register region
+      - description: CPUCP tx register region
+
+  interrupts:
+    maxItems: 1
+
+  "#mbox-cells":
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - "#mbox-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+
+    mailbox@17430000 {
+        compatible = "qcom,x1e80100-cpucp-mbox";
+        reg = <0x17430000 0x10000>, <0x18830000 0x10000>;
+        interrupts = <GIC_SPI 28 IRQ_TYPE_LEVEL_HIGH>;
+        #mbox-cells = <1>;
+    };