dt-bindings: firmware: thead,th1520: Add support for firmware node

The kernel communicates with the E902 core through the mailbox
transport using AON firmware protocol. Add dt-bindings to document it
the dt node.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Acked-by: Drew Fustini <drew@pdp7.com>
Link: https://lore.kernel.org/r/20250311171900.1549916-2-m.wilczynski@samsung.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/firmware/thead,th1520-aon.yaml

@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/firmware/thead,th1520-aon.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: T-HEAD TH1520 AON (Always-On) Firmware
+
+description: |
+  The Always-On (AON) subsystem in the TH1520 SoC is responsible for managing
+  low-power states, system wakeup events, and power management tasks. It is
+  designed to operate independently in a dedicated power domain, allowing it to
+  remain functional even during the SoC's deep sleep states.
+
+  At the heart of the AON subsystem is the E902, a low-power core that executes
+  firmware responsible for coordinating tasks such as power domain control,
+  clock management, and system wakeup signaling. Communication between the main
+  SoC and the AON subsystem is handled through a mailbox interface, which
+  enables message-based interactions with the AON firmware.
+
+maintainers:
+  - Michal Wilczynski <m.wilczynski@samsung.com>
+
+properties:
+  compatible:
+    const: thead,th1520-aon
+
+  mboxes:
+    maxItems: 1
+
+  mbox-names:
+    items:
+      - const: aon
+
+  "#power-domain-cells":
+    const: 1
+
+required:
+  - compatible
+  - mboxes
+  - mbox-names
+  - "#power-domain-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    aon: aon {
+        compatible = "thead,th1520-aon";
+        mboxes = <&mbox_910t 1>;
+        mbox-names = "aon";
+        #power-domain-cells = <1>;
+    };