dt-bindings: i2c: i2c-mux: document general purpose i2c-mux bindings

Describe how a general purpose multiplexer controller is used to mux an
i2c bus.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>