dt-bindings: mux: add adi,adgs1408

Adding documentation for adgs1408/1409 multiplexer. The bindings
follow the standard SPI and mux bindings and do not require any
additional custom properties.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[peda: reword idle-state to non-array for singular mux controller]
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>