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>