hwmon: (amc6821) add support for tsd,mule

hwmon: (amc6821) add support for tsd,mule

Theobroma Systems Mule is an MCU that emulates a set of I2C devices,
among which is an amc6821 and other 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 (amc6821: reg 0xff)

Implement "tsd,mule" compatible to instantiate the I2C-mux platform device
when probing the amc6821.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Farouk Bouabid <farouk.bouabid@cherry.de>
Message-ID: <20240906-dev-mule-i2c-mux-v8-4-dbd28a150e41@cherry.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>