dt-bindings: mailbox: Add MT8196 GPUEB Mailbox

The MediaTek MT8196 SoC includes an embedded MCU referred to as "GPUEB",
acting as glue logic to control power and frequency of the Mali GPU.
This MCU runs special-purpose firmware for this use, and the main
application processor communicates with it through a mailbox.

Add a binding that describes this mailbox.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Documentation/devicetree/bindings/mailbox/mediatek,mt8196-gpueb-mbox.yaml

@@ -0,0 +1,64 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/mediatek,mt8196-gpueb-mbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MediaTek MFlexGraphics GPUEB Mailbox Controller
+
+maintainers:
+  - Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
+
+properties:
+  compatible:
+    enum:
+      - mediatek,mt8196-gpueb-mbox
+
+  reg:
+    items:
+      - description: mailbox data registers
+      - description: mailbox control registers
+
+  reg-names:
+    items:
+      - const: data
+      - const: ctl
+
+  clocks:
+    items:
+      - description: main clock of the GPUEB MCU
+
+  interrupts:
+    items:
+      - description: fires when a new message is received
+
+  "#mbox-cells":
+    const: 1
+    description:
+      The number of the mailbox channel.
+
+required:
+  - compatible
+  - reg
+  - reg-names
+  - clocks
+  - interrupts
+  - "#mbox-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/mediatek,mt8196-clock.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/interrupt-controller/irq.h>
+
+    mailbox@4b09fd80 {
+        compatible = "mediatek,mt8196-gpueb-mbox";
+        reg = <0x4b09fd80 0x280>,
+              <0x4b170000 0x7c>;
+        reg-names = "data", "ctl";
+        clocks = <&topckgen CLK_TOP_MFG_EB>;
+        interrupts = <GIC_SPI 608 IRQ_TYPE_LEVEL_HIGH 0>;
+        #mbox-cells = <1>;
+    };