dt-bindings: iio: multiplexer: Convert io-channel-mux bindings to DT schema

Straight conversion of the txt file using the mux-consumer.yaml
binding now that is available.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Peter Rosin <peda@axentia.se>
[robh: Drop quotes and $ref for mux-controls]
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210522112908.1611389-3-jic23@kernel.org