dt-bindings: mailbox: Add ASPEED AST2700 series SoC

Introduce the mailbox module for AST27XX series SoC, which is responsible
for interchanging messages between asymmetric processors.

Signed-off-by: Jammy Huang <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Andrew Jeffery <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

Documentation/devicetree/bindings/mailbox/aspeed,ast2700-mailbox.yaml

···
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/aspeed,ast2700-mailbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ASPEED AST2700 mailbox controller
+
+maintainers:
+  - Jammy Huang <[e-mail]>
+
+description: >
+  ASPEED AST2700 has multiple processors that need to communicate with each
+  other. The mailbox controller provides a way for these processors to send
+  messages to each other. It is a hardware-based inter-processor communication
+  mechanism that allows processors to send and receive messages through
+  dedicated channels.
+
+  The mailbox's tx/rx are independent, meaning that one processor can send a
+  message while another processor is receiving a message simultaneously.
+  There are 4 channels available for both tx and rx operations. Each channel
+  has a FIFO buffer that can hold messages of a fixed size (32 bytes in this
+  case).
+
+  The mailbox controller also supports interrupt generation, allowing
+  processors to notify each other when a message is available or when an event
+  occurs.
+
+properties:
+  compatible:
+    const: aspeed,ast2700-mailbox
+
+  reg:
+    items:
+      - description: TX control register
+      - description: RX control register
+
+  reg-names:
+    items:
+      - const: tx
+      - const: rx
+
+  interrupts:
+    maxItems: 1
+
+  "#mbox-cells":
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - reg-names
+  - interrupts
+  - "#mbox-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+
+    mailbox@12c1c200 {
+        compatible = "aspeed,ast2700-mailbox";
+        reg = <0x12c1c200 0x100>, <0x12c1c300 0x100>;
+        reg-names = "tx", "rx";
+        interrupts = <GIC_SPI 103 IRQ_TYPE_LEVEL_HIGH>;
+        #mbox-cells = <1>;
+    };