LF-824: doc: Add documentation of seco_mu driver

Signed-off-by: Franck LENORMAND <franck.lenormand@nxp.com>
(cherry picked from commit f23aa19e875f7ca786c50116ad1b4b7c04625628)

Documentation/devicetree/bindings/arm/freescale/fsl,seco_mu.yaml

@@ -0,0 +1,78 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/freescale/fsl,seco_mu.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NXP i.MX8 SECO MU driver
+
+maintainers:
+  - Aisheng Dong <aisheng.dong@nxp.com>
+
+description: |
+  Create char devices in /dev as channels of the form /dev/seco_muXchY with X
+  the id of the driver and Y for each users. It allows to send and receive
+  messages to the SECO.
+
+properties:
+  compatible:
+    enum:
+      - fsl,imx-seco-mu
+
+  mboxes:
+    description:
+      List of <&phandle type channel> - 4 channels for TX, 4 channels for RX,
+      1 channel for TXDB (see mailbox/fsl,mu.txt)
+    maxItems: 9
+
+  mbox-names:
+    items:
+      - const: tx0
+      - const: tx1
+      - const: tx2
+      - const: tx3
+      - const: rx0
+      - const: rx1
+      - const: rx2
+      - const: rx3
+      - const: tx_started
+
+  fsl,seco_mu_id:
+    description:
+      Identify the driver instance, used to create the channels, default to 1
+    allOf:
+      - $ref: /schemas/types.yaml#/definitions/uint32
+      - enum: [0,1,2,3]
+
+  fsl,seco_max_users:
+    description:
+      Number of channels to create, default to 2
+    allOf:
+      - $ref: /schemas/types.yaml#/definitions/uint32
+      - enum: [0,1,2,3,4,5,6,7,8,9]
+
+required:
+  - compatible
+  - mboxes
+  - mbox-names
+
+examples:
+  - |
+    seco_mu: seco_mu {
+      compatible = "fsl,imx-seco-mu";
+      mbox-names = "tx0", "tx1", "tx2", "tx3",
+             "rx0", "rx1", "rx2", "rx3",
+             "tx_started";
+      mboxes = <&mu 0 0
+          &mu 0 1
+          &mu 0 2
+          &mu 0 3
+          &mu 1 0
+          &mu 1 1
+          &mu 1 2
+          &mu 1 3
+          &mu 2 0>;
+
+      fsl,seco_mu_id = <1>;
+      fsl,seco_max_users = <4>;
+    };