dt-bindings: mux-adg792a: document devicetree bindings for ADG792A/G mux

Analog Devices ADG792A/G is a triple 4:1 mux.

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>