dt-bindings: i2c: add support for tsd,mule-i2c-mux

Theobroma Systems Mule is an MCU that emulates a set of I2C devices,
among which devices that are reachable through an I2C-mux. The devices
on the mux can be selected by writing the appropriate device number to
an I2C config register.
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Farouk Bouabid <farouk.bouabid@cherry.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>

Documentation/devicetree/bindings/i2c/tsd,mule-i2c-mux.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/i2c/tsd,mule-i2c-mux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Theobroma Systems Mule I2C multiplexer
+
+maintainers:
+  - Farouk Bouabid <farouk.bouabid@cherry.de>
+  - Quentin Schulz <quentin.schulz@cherry.de>
+
+description: |
+  Theobroma Systems Mule is an MCU that emulates a set of I2C devices, among
+  which devices that are reachable through an I2C-mux. The devices on the mux
+  can be selected by writing the appropriate device number to an I2C config
+  register.
+
+
+      +--------------------------------------------------+
+      | Mule                                             |
+  0x18|    +---------------+                             |
+  -------->|Config register|----+                        |
+      |    +---------------+    |                        |
+      |                         V_                       |
+      |                        |  \          +--------+  |
+      |                        |   \-------->| dev #0 |  |
+      |                        |   |         +--------+  |
+  0x6f|                        | M |-------->| dev #1 |  |
+  ---------------------------->| U |         +--------+  |
+      |                        | X |-------->| dev #2 |  |
+      |                        |   |         +--------+  |
+      |                        |   /-------->| dev #3 |  |
+      |                        |__/          +--------+  |
+      +--------------------------------------------------+
+
+
+allOf:
+  - $ref: /schemas/i2c/i2c-mux.yaml#
+
+properties:
+  compatible:
+    const: tsd,mule-i2c-mux
+
+required:
+  - compatible
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    i2c-mux {
+        compatible = "tsd,mule-i2c-mux";
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        i2c@0 {
+            reg = <0x0>;
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            rtc@6f {
+                compatible = "isil,isl1208";
+                reg = <0x6f>;
+            };
+        };
+    };
+...
+