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.
Signed-off-by: Farouk Bouabid <farouk.bouabid@xxxxxxxxx>