dt-bindings: irqchip: Describe the IMX MU block as a MSI controller

I.MX MU supports generating IRQs by writing to a register.

Describe its use as a MSI controller so that other blocks (such as a PCI EP)
can use it directly.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220922161246.20586-5-Frank.Li@nxp.com

Documentation/devicetree/bindings/interrupt-controller/fsl,mu-msi.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/interrupt-controller/fsl,mu-msi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Freescale/NXP i.MX Messaging Unit (MU) work as msi controller
+
+maintainers:
+  - Frank Li <Frank.Li@nxp.com>
+
+description: |
+  The Messaging Unit module enables two processors within the SoC to
+  communicate and coordinate by passing messages (e.g. data, status
+  and control) through the MU interface. The MU also provides the ability
+  for one processor (A side) to signal the other processor (B side) using
+  interrupts.
+
+  Because the MU manages the messaging between processors, the MU uses
+  different clocks (from each side of the different peripheral buses).
+  Therefore, the MU must synchronize the accesses from one side to the
+  other. The MU accomplishes synchronization using two sets of matching
+  registers (Processor A-side, Processor B-side).
+
+  MU can work as msi interrupt controller to do doorbell
+
+allOf:
+  - $ref: /schemas/interrupt-controller/msi-controller.yaml#
+
+properties:
+  compatible:
+    enum:
+      - fsl,imx6sx-mu-msi
+      - fsl,imx7ulp-mu-msi
+      - fsl,imx8ulp-mu-msi
+      - fsl,imx8ulp-mu-msi-s4
+
+  reg:
+    items:
+      - description: a side register base address
+      - description: b side register base address
+
+  reg-names:
+    items:
+      - const: processor-a-side
+      - const: processor-b-side
+
+  interrupts:
+    description: a side interrupt number.
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  power-domains:
+    items:
+      - description: a side power domain
+      - description: b side power domain
+
+  power-domain-names:
+    items:
+      - const: processor-a-side
+      - const: processor-b-side
+
+  interrupt-controller: true
+
+  msi-controller: true
+
+  "#msi-cells":
+    const: 0
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - interrupt-controller
+  - msi-controller
+  - "#msi-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/firmware/imx/rsrc.h>
+
+    msi-controller@5d270000 {
+        compatible = "fsl,imx6sx-mu-msi";
+        msi-controller;
+        #msi-cells = <0>;
+        interrupt-controller;
+        reg = <0x5d270000 0x10000>,     /* A side */
+              <0x5d300000 0x10000>;     /* B side */
+        reg-names = "processor-a-side", "processor-b-side";
+        interrupts = <GIC_SPI 191 IRQ_TYPE_LEVEL_HIGH>;
+        power-domains = <&pd IMX_SC_R_MU_12A>,
+                        <&pd IMX_SC_R_MU_12B>;
+        power-domain-names = "processor-a-side", "processor-b-side";
+    };