i2c: muxes: add support for tsd,mule-i2c multiplexer
Theobroma Systems Mule is an MCU that emulates a set of I2C devices,
among which an amc6821 and 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).
This driver is expected to be probed as a platform device with amc6821
as its parent i2c device.
Add support for the mule-i2c-mux platform driver. The amc6821 driver
support for the mux will be added in a later commit.