dt-bindings: iio: io-channel-mux: document io-channel-mux bindings

Describe how a multiplexer can be used to select which signal is fed to
an io-channel.

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