If it is properly defined in devicetree, the emulated AMC6821 should be
an i2c device, possibly sitting behind an i2c multiplexer, not a
platform device.
The emulated AMC6821 and the Mule I2C mux are both reachable using I2C address (0x18), and hence the use of MFD as the mux only uses one I2C register that is not used by AMC6821.